Their model is to sell USDT and maintain a balance that at least equals as much in USD as they have issued USDT. In reality, there's no requirement for them to maintain a specific ratio and their model is built on trust. There are no third party verifications of their bank balances, even though they say they have audited balances.
They also have no banking relationships with US based banks so if you tried to actually convert USDT and withdrawal, who knows if it would actually work. I like their concept, but they have many challenges because of their banking issues.
So just release more USDT supply to control the price. Do they have a finite tokens supply or unlimited to match federal reserves' printing appetite in quantitative easing?