To be honest I don't really get how physics can describe nothing. I think I'll try reading the book version of his argument, which I imagine is more detailed.

I think there are several potential limitations with how far we can speculate on the origins of the universe using modern physics.

First, we know that modern physics is incomplete. For example, the best mathematical models we have for describing nature at the macro scale (relativity) and the really small scale (quantum mechanics) are fundamentally incompatible. We don't have an empirically validated unified theory of everything yet.

Second, even when physics is "complete", We'll never have a model that we know is absolutely true. The best we can hope for is a model we can't falsify with an conflicting experiment or observation. But that was true for Newtonian physics as well. For a while.

Finally, by definition, science is limited to approaching the truth through experiments and observations in our universe. It seems a bit of a stretch to imagine we can use science to verify what is true outside (or "under") our universe.

I admit this isn't at all my area of expertise so I might be missing something. I'll add Kraus's book to my reading list.