Well, a Turing complete computer is about as abstract as you can get, and so is the concept of memory, regardless of the natural laws of some outer universe, it'd probably be a good starting point to consider a turing computer except with limited memory.

The point is not to prove one way or another whether the universe is a simulation, just like there is no point trying to prove the law of conservation of energy as universal for all forms of phenomena, discovered and undiscovered.

It's just if we can show that all known phenomena can be the result of a very limited simulation, then when analyzing new phenomena, we can also expect it to be runnable on a very limited simulation, too.

It's the same as assuming that the law of conservation of energy would apply to any new phenomena we discover. There is no proof, but it's a good starting point.