An important distinction between Terra (what is being described there) and Tether (what this thread is discussing), is that Tether claims to be backed by real assets.
Also the price of a stablecoin can shift from it's peg, even if it has 100% backing, when there is latency/congestion/spread/volume for the arbitrage trade.
That said, there is a whole heap of plenty of evidence that USDT is not backed by dollars 1:1.