Tether (and the various exchanges that use it) has enough liquidity to process withdrawals, and likely has enough slack in the system to process periods of higher then normal withdrawal amounts.

As long as Tether has some liquidity, things will keep operating as normal. However, if the amount of money withdrawn is greater then the amount deposited, eventually Tether's reserves will dry up. Once that happens, anyone with outstanding USDT will be stuck with a worthless asset. Only Tether knows how close we are to that point, and they aren't saying.