Presumably at some point they would figure out that their skybox is fixed in a way that either doesn't actually follow any rules of basic trignonometry, and thus is a simulation/projection, or, they'd find that it does follow the rules of trig, and that would prove that their world has 0 curvature.

If Eratosthenes had done what he'd done, in that world, he would have pretty definitively concluded that the world is indeed provably flat, and is potentially an infinite plane, around ~ 220 BCE.

Also: How do 'days' work, on an infinite flat plane?

This is actual flat earther levels of nonsense if you think about it for like, 15 seconds.

Here's a better world building idea:

Build your alt history or fantasy or post apoc world (why not all three?) on a world that is totally covered in a very thick atmosphere, or has some property by which there is essentially always 100% cloud cover, basically everywhere.

It can be flat, or not.

Astronomy and thus our whole tech tree path for like half of science is now essentially impossible.

It would follow a totally different pattern of development of thought and ideas, in many ways.