A lot of humanoid factors locked in over 400 million years ago.
First of all "it's just convergent evolution" actually is not a bad answer.
There are tons of reasons I will discuss below about why humanoids are the most likely body plan for sapient life, but by introducing something along the lines of a lobe fish, you can already have enough evolutionary factors locked in to pretty much guarantee humanoid life.
Let's delve into this a bit. Intelligence has evolved a lot of times here on Earth. Many species of birds, dolphins, whales, and cephalopods demonstrate level of intelligence comparable to early hominins and all beat our ancestors to this level of intelligence by million years... but despite evolving intelligence, means of communication, and basic tool use similar to what we had just a few million years ago, none of these animals took the final steps that we humanoids did. These organisms never developed tool using beyond that of found or slightly modified objects.
I know sapience means a lot of things depending on who you ask but for the remainder of this answer, I will used sapient to mean capable of forming technologically advanced societies, simply for lack of a better word.
Sapience can't evolve in aquatic life
It has been argued that several marine animals are at least as smart as humans, but none of them seem to be able to wield any technology more advanced than a sharp rock. In order for any animal to get past the early stone age, it needs to master fire. This basic skill is required for ceramics, metallurgy, cooking, glue making, etc... practically every human technology that makes us more advanced than chimpanzees requires at some point in its production, the application of fire. So, all the smart stuff that evolves in the water never becomes an industrialized society because they can't make the tools of the trade.
Seeding Caveat: DNA is rarely preserved for more than 1-2 million years; so, when comparing the fossil records of animals 400 million years ago, there are no genetics to compare, just shapes and forms embedded into stone. This means that if your aliens are seeding a young world that already has complex aquatic life, but no dominant terrestrial fauna, they could decide to drop in something like a lobe fish. This way, they could make sure that thier seed organism wins the land race, but the seed life happened so long ago that by the time sapient land life emerges that any sketchiness in the fossil records would fall into the "to be expected" category.
Sapience can't evolve in small life
This is more than just a limitation on brain size. It goes right back to the fire issue. Small fires made by small creatures behave very differently than large fires made by large creatures. Fire is not practical for a stone age creature much smaller than humans because it takes so much fuel to keep them burning. Containing the heat of a fire to concentrate it for things like ceramics, smelting, and baking also requires a certain minimum size to be effective. This means that even if you had a super smart squirrel like creature, the effort it would take to gather the amount of wood it would take to smelt ore or fire clay would be proportionally immense. A wood kiln for example consumes a minimum of about 1 cubic meter of wood. This is true if you are firing a single small cup or a whole tea set because it takes a continuous burn and trapping of heat to get up to and maintain temperatures without shattering your clay and make it bind.
Seeding Caveat: Animal size evolves up and down very quickly. There is no real controlling for the ability to introduce a seed organism and ensuring it becomes big, but you can introduce a seed organism and expect that some of its descendants could be big, and those will be the most likely to develop sapience.
Sapience will be extremely rare in species with more or less than 4 primary appendages.
Bilateral symmetry is the preferred body plan of nearly all complex life forms. While evolution likes to start off with asymmetry, the prevalence of bilateral symmetry in complex life on Earth seems to indicate a strong preference for this across all evolutionary paths. This means that body plans with 1 or 3 primary appendage is unlikely in more complex life forms. 2 primary appendages generally won't work with terrestrial life because if terrestrial life evolved with only 2, there is guaranteed to be a lot more evolutionary pressures to makes those into specialized feet than hands. This means that in the vast majority of cases, sapient organisms need at least 4 appendages to have both specialized walking and tool using appendages.
But... as I said previously, small life and aquatic life does not make good sapient life, which leaves the question about if more than 4 limbs is likely. Appendages are heavy and take up a lot of resources. The bigger an animal gets, the more expensive extra limbs becomes in terms of the square cube law and pure energy requirements to maintain them. This is why pretty much all large terrestrial life on Earth are tetrapods. It's a perfect compromise between have enough libs to get the job of surviving done without having so many limbs that you are wasting resources. Just like every branch of large terrestrial life is tetrapods on Earth, it seems feasible that this body plan will be the norm on other worlds (at least those with similar gravity).
Seeding Caveat: Here on Earth, we pretty much locked into the tetrapod body plan very early on. Tetrapods emerged even before we moved to land, and we never once had a mutation lead to any significant populations of children with more or less primary appendages. So, if you seed with a tetrapod, they will likely stay tetrapods.
Sapience requires a World with Earth like mass
A heavy world requires smaller life which I've already explained comes with a lot of limiting factors, but what about smaller worlds? Well as it turns out smaller worlds become dead worlds much faster than big ones. The core cools, the magnetic fields disappear and the atmosphere blows off into space. This means smaller worlds won't give evolution enough time to get to intelligent life.
Seeding Caveat: Very Earth like worlds may be a prerequisite of your progenitor species. While it might be possible for life to evolve in lots of strange places, if we humans wanted to seed another world, we'd have to pick something VERY Earth like. Earth like atmosphere, hydrosphere, gravity, mineral composition, temperature, year length, axis tilt, EM field, etc... if your alien progenitors are seeding the universe with THIER primitive life forms, then they will have to be a lot more picky than natural evolution meaning that all worlds where thier seed organism establish a foothold must provide very similar evolutionary pressures to thier homeward.
Sapience requires something like hands
While there are some pretty smart birds and bears with relatively dexterous feet, none of these animals can manipulate things with nearly the ease and precision that a humanoid can. This will make getting past primitive technology very hard for those without the hands to perform intricate tool use. While I don't think exactly 5 fingers with 3 joints each will be a given, I do think that something with multiple flexible finger like protrusions will be necessary.
Seeding Caveat: Like Tetrapoda, fingers locked in pretty early too. While an animal could perhaps evolve with tentacle like hands, pretty much all tetrapods here on earth have 5 fingers and toes. So, even if tentacles or 6 fingers is more common among native organisms, the seed organism will likely go on to have 5 fingers. Scientists will note the mutation in the fossil records, but this will not cause any red flags, but be part of thier early evolution narrative.
Non-humanoid life gets out-competed by humanoids when it does happen
With all the selective pressure pushing towards humanoids, this means that in the rare case where another very different body plan is selected for, that body plan will typically go extinct once its world also evolves a humanoid.
Seeding Caveat: In natural evolution, there is no guarantee that humanoids will ever evolve to compete with native sapient life, but if you introduce just about any tetrapod ancestor, the humanoid form becomes a lot more likely
What all of this means is that there is a LOT of pressure for convergent evolution in sapient life. Just like the general form of a cat/dog/hyena etc. is selected for over and over again for terrestrial predators, we will see something generally humanoid selected for over and over again in sapient life. There will be a lot of variance, sure, but by-in-large they will nearly always be upright terrestrial tetrapods of roughly human size with 2 specialized hands and 2 specialized feet... aka humanoids. Especially if you seed the planet with a tetrapod to get things started.