You state that they have abundant supplies of LEGO of all types. I am assuming this means LEGO Technic parts as well.
LEGO Technic includes many types of gears, levers, axles, wheels, pistons, air compression and pneumatics, shock absorbers, rack-and-pinion sets, universal transfer boxes, springs, and so on.
I am also assuming that they may have lost our current knowledge and science, but they have at least retained a modicum of our intellectual capability. They would be able to think and learn, communicate, and to use tools.
So do not think in terms of LEGO pieces, think in terms of LEGO knowledge.
The greatest contribution to defence that LEGO would provide is knowledge about mechanics, levers, gear ratios, wheels, and knowledge in using these devices. Children growing up would learn about such concepts as gear reductions, wheels and axles, mechanical advantage, simple machines, and other engineering concepts. So although this civilization might not have electricity, steam power, metalurgy, advanced chemistry, or scientific knowledge, they would definitely have mechanical engineering knowledge. Copying the LEGO pieces into larger mechanical devices and apparatus would be absolutely expected. If they can build it in LEGO, then they can build it in wood. These Legolanders would have, at the least, an understanding of levers and basic catapults. They would definitely have carts and other axle transportation. Certainly, they would have the princciples of compressed air and pistons. They would have access to springs and the knowledge behind the concepts of a spring.
So the LEGO advantage would not be in building the weapons out of LEGO, but of building the prototypes out of LEGO, and transfering the design to wood, clay, and such. It is the learning and knowledge of engineering that LEGO would provide - a head start, as it were, on mechanics, just like it does for children today.
EDIT A Note on Glue
Even the neolithics had very strong glues,that could be used to bond LEGO structures.
Hide glue was once one of the most common glues in use. Hide glue was
the strongest glue available before modern chemistry brought modern
glues to the public. But don't let the primitive aspect of the glue
fool you, hide glue is a very strong glue.
EDIT addendum - summarizid my comments into one place.
The following assumes they have not regressed (devolved) in intellectual capacity, but that they are just lacking knowledge and skills, and certainly no metals or metal working, no saws, steel tools, or such (all ores used up, maybe? Certainly, one thousand years and we will run out of metals).
Ways to scale up LEGO Technic parts (once they know what to make, how to make it is just human ingenuity):
Humans have been hollowing out logs since eternity. Once they have the concept, they could easily cut a log in half using stone wedges, hollow it out using sharp stone tools, and patch it back together again, just like LEGO. Seal the ends using the hide glue I just referenced, and pitch, and you have the makings of a pneumatic cylinder. Alternately, make the cylinder from clay. "The Mesopotamians introduced the world to clay sewer pipes around 4000 BCE, with the earliest examples found in the Temple of Bel at Nippur and at Eshnunna, " en.wikipedia.org/wiki/Plumbing#Water_pipes
Springs can be made from soaking sapling wood and curling it into a spiral. beds.org/blog/… Okay, that's a stretch, but they already HAVE the springs - in the LEGO pieces. Maybe not individually super powerful, after all LEGO was purposely 'mellowed down' so as not to be harmful (non violence has always been the LEGO mantra), but gang dozens together? Yes, a powerful short-range dart gun against a non-armored person.
Gears are easily made from wood, carved using stone tools. 'Early examples of gears date from the 4th century BC in China (Zhan Guo times – Late East Zhou dynasty), which have been preserved at the Luoyang Museum of Henan Province, China. ' en.wikipedia.org/wiki/Gear
Gears could be made from ceramics and even glass, for strength and longevity. Moulded and shaped glass gears would probably survive hundreds of years longer than metal gears. ceramics.org/about/what-are-engineered-ceramics-and-glass/… I wouldn't use one in a multi-ton press or a car engine, (although ceramic gears are being made with the strength of steel - http://www.roccera.com/ceramics/net-shape-mfg/) but in an unpowered society?
A note on LEGO strength itself, (AKA paying homage to the laminated interlocked LEGO solid flat ABS base plate, as opposed to the LEGO brick itself):
When most people think of a LEGO wall, they think in terms of building the wall traditionally, with one LEGO brick stacked on top of another, vertically. This is the weakest way to build a LEGO wall. Because of the nature of its structure, LEGO bricks are very strong in compressiion vertically, with the forces following down the 'tubes' inside. Even stronger is the thin solid ABS LEGO base plate, that LEGO structures are built upon. So the strongest configuration for the wall is to build it with the LEGO bricks sideways, several bricks deep, interlaced, and a large laminated and interlocked LEGO flat base plate structure sandwiching it. I tried it. It is very strong, will even resist hammer blows.
HOW TO: Lay out a layer of LEGO flat base plates, flat on the ground, twelve feet wide and as long as you want the wall. Place another (laminated) layer of flat base plates on top of this, interlocked and staggered. Repeat for several layers. Then, place long (32 bump) lego pieces side by each, ends staggered and overlapped, on the bases, until the entire base is covered in a solid layer. Build another layer, using the long blocks, only at right angles. Then, another and another, until you have quite a thick base. Laminate several layers of LEGO flat base plates on top, again staggered and interlocked. Now, you have a very large, solid, flat patform that is entirely inerlocked and integrated, that you could probably drive a transport over. It can be as thick and large as one cares to make it, given enough pieces. Raise the entire structure vertically, so it is now at right angles to the ground. Prop it up using wooden posts. An almost solid twelve foot high ABS wall perhaps a foot or more thick, with a solid flat inner and outer surface, ablatively inpenetrable by anything the BFs would have. Cover it with animal hide for UV protection, although I am not sure this is needed. I have LEGO pieces from the 60's (yes, I am that old - I was one of the first kids in Canada to play with LEGO circa early 1962), that have not deteriorated in the slightest. For even more strength, glue the inner layers of bricks together using hide glue. Three layers of flat base plates, stacked one on top of each other, are almost impossible to seperate without a sharp object to pry the layers apart. You will not, can not, stab a sharp knife all the way through five layers of flat base plates when the grain is alternated without powered assistance.
Not only could these large laminated sheets be used for walls, they could be used for shields, cart bottoms and sides, and even cut in a circle for wheels, and maybe gears. Sharp stone edges would eventually shape the LEGO, and I am sure if they had baked clay, someone would quickly learn that shattered pottery, especially if glazed, is very sharp and strong. If you have ever cut yourself on broken glass, you know exactly HOW sharp it is.
Since they can be made any width, any length, any thickness, given enough LEGO, they can be made into beams as thick as a log, perfectly straight and dimensional, to reinforce the wall (grain at right angles to the grain of the wall). Exceptionally strong if the base plate laminations are oriented perpendicular to the applied force, i.e. sideways, 'on edge'. If you have enough flat base plates and 32 bump pieces (I do), and try this, you might be amazed at the strength.
If you do indeed have enough LEGO flat base plates, you could even make roads, maybe even cover the entire ground surrounding your settlement with one huge LEGO laminated base apron, then build pyramids five or six blocks high with LEGO slopped and pointed roof pieces, to make life very difficult for barefoot attackers. It would be one huge trip hazzard, very awkward to get one's footing.
Really creative LEGO builders would use these laminated planks, beams, thick sheets and platforms to build fortified carts to use on the LEGO platform roads to journey into the forest. A bicycle made of laminated ABS LEGO flat bases on edge, maybe?
The trick is to think 'Use it sideways, not in the traditional direction. Use laminated base plates, not just the bricks. Interlock layers at right angles, not just in one direction.'
EDIT research into acetone
Acetone, AKA nail polish remover, is an excellent glue for ABS and LEGO. My research has discovered that acetone occurs naturally, in the human body, and in nature. It is easlily made from egg shells and vinegar, for instance. It is also present in very bad wine, and in the urine of some uncontrolled diabetics. Some people on an unmonitored Keto diet expell it in their sweat. So it is very much within reasonable conjecture that the Legolanders, in that thousand years, would have accidentally discovered acetone as a glue for LEGO pieces. It is not a high tech product, well within the technology level of the neolithic period.
It was first identified by an alchemist in the 1600's.