This happens all the time in the stock market. Look at the SPY stock which tracks the S&P 500 index. At everypoint in time, anyone can compute exactly what value this stock should have. Any deviation from this consistent and known price point is immediately traded away by arbitrators.
Does Tether actually hold one real United States Dollar for every USDT they issue? If so, they sure haven't convincingly demonstrated it by continually lying about being regularly audited (they have never published a real audit) and refusing to allow anyone to redeem their USDT with Tether to get back a USD.