I've only followed Archegos from afar, but I see your point. Basically it all boils down to people's trust in Tether, which if lost could cause a ripple effect and massive dump of USDT.

I think there are just too many unknowns to predict the market-wide impact, but Archegos is a good case study (in a way so is Ripple/XRP who went through a somewhat similar dump for different reasons a few weeks ago).