That's a misunderstanding of history. USD used to be backed by silver and all bills said "in silver payable to the bearer on demand". It was only relatively recently that the practice ended.
It doesn't really invalidate his statement though. The USD is not backed by anything (whether it was in the past or not is irrelevant), and aside from the fact that it's commonly accepted, has no more intrinsic value than Bitcoin.
As long as 300 million people are required to convert a fraction of their income to USD for taxes, its value can't go to zero. Bitcoin's value is purely an article of faith so far.
https://upload.wikimedia.org/wikipedia/commons/7/70/US-%245-...