So there are many iterations of compounded phantom value folded into the market cap of BTC. If tether is found insolvent, essentially 80% of the value of BTC disappears overnight.
The entire value of the crypto market depends on people's belief that one tether is worth 1USD. Or so the theory goes.
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.
What happens when that safety net vanishes as hoards of people are running off the cliff?
Edited to add:
This isn't really a theoretical issue, either. Risk is part of the price of a security. Something that has a high risk, may be valued less by investors. Often other investments will carry the same amount of reward but less risk.