I really find it hard to imagine that the caveman of the "paleolithic" age will actually know how to operate something as advanced as a spaceship?
As defined here these guys only know how to use rocks, and before we could even go to space, it took us a lot of years to do so. A LOT.
It seems like most people here agree that someone in that age or era will actually learn how to operate something as advanced as a spaceship without any help whatsoever, and will eventually learn it with just the use of manuals?
I'm a computer scientist, I have created systems and I will proudly tell you guys I cannot fly a plane even with a user's manual, I will actually need help from someone to teach me how to fly a plane, and learn from them on how to maintain that plane.
Another example can a severely secluded ethnic tribesman who was born and lived for 80 years in the forest, use a TOUCH SCREEN cellphone?
Now tell me, how in the world can a caveman of "paleolithic" age fly a ship?
The most logical reason that comes to my mind is that, the cavemen will be held either as lab rats, carried to other planets by means of "bait", and flown through space through "space cages" so that the aliens can use the cavemen as cattle, reproduction, slaves or research purposes.
In this scenario, the aliens are indirectly helping them to fly their own spaceships, but the controls are being used by the aliens, why? for some reason that these guys only knows how to use rocks.
Now some of you will say that If these cavemen are taught, then they will know how to use the tools around them right, which is basically right, but the OP actually stated that "
How can paleolithic humans colonise the stars without direct help from aliens and still be primitive?" and teaching something, is already a direct help.
Colonizing is another thing, the cavemen may colonize a planet if left alone since they will eventually reproduce, but to colonize the stars, these humans need to learn from their abductors about the technology which they use, since these aliens technology is more advanced than ours(we can travel in space but up to just what extent right?).
It might take them more than a millennium to learn how to use the alien tech for interstellar flights, but that is IF they will not be taught on how to use them.
Again, teaching them, in any way, is a direct way of helping the cavemen.