Let's correct some misunderstandings. Yes, he is using theories to infer absolute conclusive statements, but those theories aren't "debunked" because they are unfalsifiable. Thats not how it works. If it cannot be demonstrated or proven then it's simply a model. Nothing about it is debunked besides external claims that don't align with the existing models.

It is however ridiculous that he assumes his opinion is meant to be taken as a given. I also believe in multiverse theory on top of many other concepts, but I would never be so presumptive as to state my beliefs as fact.

It doesn't sound like you know what you're talking about. It being unfalsifiable doesn't mean that it is debunked. It just means that it cannot be empirically proven over other competing interpretations, which also are probably unfalsifiable as well.

I actually don't favor many worlds interpretation, but that does not preclude it as there's no way to test and verify any model currently.

Additionally there are also other models of how a multiverse would exist. If the universe goes on for infinite, or if there are multiple bubble universes with own big bang/creation event which would give the same affect. All unfalsifiable btw because it extends outside the scope of any measurement that we can ever make.

Unfalsifiable just means we don’t have the capacity to verify, not that it can’t be the case because we can’t verify it.