Your point has nothing to do with fiat.
It applies equally well to a fiduciary coin with a promise that 1 gcoin = 1g of gold or whatever.
It applies equally well to a fiduciary coin with a promise that 1 gcoin = 1g of gold or whatever.
However, it does not necessarily apply to algorithmic stable coins (Terra) or crypto-backed overcollateralised coins (DAI). That's why I said "fiat-backed".
Deposits at a regular bank aren't backed by fiat. The bank promises to pay you back in fiat, but they don't keep dollar bills in a vault. The promise is backed with all the bank's assets. Ideally, they have a think equity cushion, so that even if the assets take a hit, the promises don't.