It’s not a proof of possible/impossible universes, it’s a counterexample to the argument that infinite universes must necessarily mean that there’s a universe with anything in it. It disproves that there must be a universe with X in it because there are infinite universes. It does not disprove that there isn’t a universe with X in it.

It makes no claims that the multiverse must necessarily be constrained, but it does show that you can’t simply assume that the multiverse must be unconstrained. That needs to be proven first. And there are compelling arguments to assume that said constraints do exist (in some way).