Earlier quoted context omitted.
You don't have to hold dollars to spend dollars, is the only thing I can think of.
Coinbase holds the dollars for you. It's arguable to what extent bitcoin even factors in here after your initial deposit, since everything is under the control of Coinbase. You are effectively holding a note called "Coinbase bitcoin" that fluctuates in value compared to the dollar, and you get a spot exchange rate when you spend. This is not the way bitcoin was meant to be held and spent.
That seems like a meaningless assertion to make about an unregulated currency. If the free market optimizes for regulated intermediaries, that's perfectly valid.