They also state that all USDT are really supported by 1:1 real USD to USDT. Which I can't believe would be possible, will this mean they have like billions of dollars already in their reserves?

Just to illustrate, bitcoin prices has risen far beyond how much deposit USDT has gotten but USDT keeps on coming. Like for example on poloniex all trades in fiat are in USDT which means when the prices of bitcoins increased USDT was always ready to match the increase. Coincidence? Probably. But I agree with thymos, I don't think this USDT is legit all the way.