I think the point being made is that the exchange rates of any currency -- whether from currency A to currency B, or currency B to "hot dog, fries and soda" --

I think the point being made is that the exchange rates of any currency -- whether from currency A to currency B, or currency B to "hot dog, fries and soda" -- are essentially set by shared belief, not by some kind of intrinsic value, no matter what the currency is ostensibly backed by. The idea that currency needs to be backed by anything in the sense it's being used here (e.g., a tether backed by a dollar, or a dollar backed by a fixed amount of metal) is a convenient fiction that requires trusting all the entities explicitly or implicitly involved in the transaction. I'm aware bitcoin and other blockchain-based "currency" tries to expunge the need for trust, but in practice it seems like most of them succeed more in expunging accountability. The argument that state-issued currencies have value because they're the only currency you can use to pay taxes back to the state is true in some sense, but the value is still essentially arbitrary and the trust is still required. (e.g., if I owe money come April 15th, I have to trust my bank, Visa, and TurboTax, probably, even before it gets to the IRS).