The USD is backed by the US governments ability to pay it's debts, which is a legit financial instrument of value. It's not a 'fiat' currency in the sense being described here.
What? The US dollar absolutely is fiat currency, and anyone saying otherwise is simply using a non-standard alternative definition of "fiat currency" than what everyone else is using. Every economist ever will tell you that of course currency issued by a central bank that isn't backed by precious metals is a fiat currency, and that USD along with all the other major currencies count too.
The USD is a fiat currency that is worth what people (the market) values it at. The ability of the US government to pay it's debts is but one (very important) factor in that distributed calculation. It is not solely determined by that. If it were the value would rise or fall solely on the debt-to-GDP ratio - which is probably only loosely correlated - up to a point where default becomes probable and then it matters an awful lot more, like with Argentina.