I think this is basically a scam, there is no way they have enough $USD to cover all the USDT in circulation, the people who sell last will be left with worthless USDT....
In my opinion it is kind of pointless to try to peg any crypto to $USD. $USD is on a slow path to hyper inflation (much like most fiat currencies that are controlled by central banks), it will eventually fail, why would you want to peg a currency to another currency that you know will ultimately become worthless...