If USDT would go to 0 instantly many things would happen (a few of the biggest BTC fiat markets would not be able to operate, such as binance's main BTC market). As a practical example big traders would obviously sell their tether straight into crypto on those markets. But since they don't want to be exposed to that much crypto they would sell the same amount on other exchanges (such as Coinbase). In these chaotic events the biggest traders will fly to safety for most on their books (actual fiat, or hedged via derivatives).

But that's not the potential issue being described: Tether is supposed to be worth 1 dollar. A lot of big crypto markets run on tether. If it turns out that a lot of crypto was bought with synthetic dollars (tether) that turned out to be a lot less than a dollar, that would mean the price of many crypto projects got to where it is by imaginary money. In another word: overpriced.