With a bank, you are lending cash to them when you make a deposit. You are taking a risk that the bank’s credit won’t be good, and you will lose your money, so you should require interest to compensate for that. Deposit insurance changes this only a little. But it’s not like there are zero assets backing the accounts: there are still enough to cover expected liquidity needs. The credit risk mentioned early is that his expectation will fall short of reality.
But owning tether wasn’t supposed to invoke any credit risk at all. It was a secured loan: if you bought a tether, you had sole claim on a dollar in a bank account.
By the way, banks buy and sell secured loans all the time. Mortgages are a particularly common example in retail banking. Although usually, secured lending is used to convert a low liquidity asset, like a house, into a liquid one, like cash. Tether makes no sense economically because it converts one liquid asset into another.