This is silly. He says that IF the belief the dollar will be worthless is ubiquitous, then it will become worthless. That's a function of the market, a true fact.

The economic power and military are already calculated into that. It makes it very unlikely that people will think the US dollar will become worthless. But he wasn't talking about whether that belief is likely, but simply stating the true fact that IF everyone starts believing that, then demand crashes and supply skyrockets accordingly and it'll become worthless.

In other words, no different from any currency or any good on the market for that matter, including bitcoin.