De-pegged is just any value less than $1. If USDT is 50% collateralized, maybe it is worth $0.50, which is de-pegged but not worthless.
Also, Tether would try to save face, by honoring USDT redemption at 1$ as long as they can, which drains their reserves. So if they aren't able to do it anymore, it means that they don't have anything left, and USDT is now worthless.
These things are highly non-linear and bank runs can kill a bank in no time (and tether is functionally a bank).