I very much don't understand how markets and order books work, but people who do understand those things tell me pretty consistently how big of a deal it is when a money market fund loses its peg, even by a tiny amount.
USDT maintains its peg by allowing certain entities to redeem USDT 1 to 1. How much USDT's price fluctuates depends on how well that mechanism works, and market conditions.
By claiming certain entities can redeem 1:1. No one has ever documented a Tether redemption. They have a very tiny number of clients and the terms say they don’t have to redeem at all.