Approximately half a billion USD are traded for USDT every single week though... and have been for a number of years now: https://cryptowat.ch/charts/KRAKEN:USDT-USD?period=1w_Monday
If there was a discrepancy between the amount of USDT and USD controlled by Tether, surely that peg would be far away from $1 USD/$1 USDT right now no?
It has slipped during periods of mass hysteria in the crypto markets a few times in the last few years but has always reverted to $1 within a few days tops.