that's how it's always been. what do you think has been backing USD balances at bitfinex all these years? or for that matter, gox?

the only difference between your bitfinex USD balance and USDT is whether you can send it over a blockchain. exchanges could be run as fractional reserves and can inflate prices with unbacked trading as well---on top of backing their USD balances with "non-limit printed dollars". stablecoins don't really change anything in that regard.