If you imagine simulations we can build ourselves, such as video games, it's not hard to add something at the edge of the map that users are prevented from reaching and have the code send "this thing is massive and powerful" data to the players. Who's to say that the simulation isn't actually focussed on earth, and everything including the sun is actually just a fiction designed to fool us?
Where have you seen this?
If we're a simulation of a parent universe that is exactly like us just of it's past or an alternate past, then we likely should be able to achieve simulating our own universe within ourselves. Otherwise we're not actually a simulation.
There's another line of counter argument that various results in QM and computing theory would suggest that it's mathematically impossible for the universe to be simulated on a computer (i.e. the parent universe would have to look very different from ours vs ours in the future). But I don't recall the arxiv paper.
Yes, this is a MASSIVE and COMPLETELY UNTESTABLE if
Everything about simulation theory is like, science-hostile or something it seems.