I have a sort of problem with it, because I think you're nitpicking. When I say "our" universe, I mean the universe as we understand it now. Surely, if we are in a simulation, our "universe" is modeled after the original society's universe, probably has the same laws of physics and whatnot. I didn't mean we originated from the same universe as our creators, because that wouldn't make sense.
I'm saying that since the universe is supposedly infinite, then our creator's universe is probably infinite as well, which means that ample resources were given within the cosmos for a civilization to pop up, survive long enough to have advanced technology, and make a simulated reality that we live in. If the universe they live in is supposedly infinite, like ours, then there's a very small chance that this isn't the case. If that simulated reality gets to the point where they can create a simulated reality, then there's an even higher chance that we're in one. Of course, most of this is just theorizing and there's' no way to prove any of it. It's just an interesting thought experiment.