Apparently not. Try spending dollars in a store (both in the U.S. and many places besides) and see if it works.
They are only worth something because people accept them as payment.
Including the U.S. government as payment for debt. If the most powerful government in the world says dollars have value, it's not so easy to lose all of that value, even without it being a backed currency.
Is he saying that dollars are the only solid currency in the world, and everybody should convert all their assets into dollars as fast as possible?
Most of the world uses non-backed fiat currency now, not just the U.S. People do convert their assets into and out of dollars (and other currencies) all the time.