"The universe is a simulation" is a very strong claim, and most of the justifications for it also support the weaker claim "the universe is computable".
"The universe is a simulation" is exactly as strong of a claim as "There is a god."
No read the wikipedia on it. They use probability to prove it's basically true. That is the argument that is hard to argue against and the one Elon goes on about. It sounds wrong but it's logically hard to show why.
The argument relies on a couple of assumptions which are not obviously true (they're not obviously false either). That's where most of the criticism of it is aimed.