I don't see how that's relevant to reasons people buy dollars and sell them to other buyers....Facts about the history of dollars may make dollars more stable compared to bitcoins, but what I'm saying is that those facts don't enter into the minds of most people as reasons they buy dollars.
I think his argument is that today bitcoins are not valuable and are grossly overpriced. Since (he claims) there is no reason for the price to go up it will continue to collapse as more people understand this certainty.
He hasn't said this, but I believe his argument assumes a certain acceptance threshold must be reached. The current prices assume that threshold will be crossed, but that as prices come down and more people exit, the threshold becomes less obtainable, and bitcoin will just peter out.
When dollars WERE backed by gold, they easily crossed this acceptance threshold. When the backing was removed, the fiat was sufficiently established. So, while the historical backing is not relevant today, it was relevant to its establishment in people's minds.