As a matter of fact that is NOT true. (speaking as a crypto hedge fund manager, active since 2013, and trading > $1B/month on all major exchanges). It used to be like that in the past (when USD and USDT where one just represented as "USD" on Bitfinex), but this changed a year or two ago. Now, USD wires are being credited as USD and USDT deposits are being credited as USDT. And there is a USDT/USD market on Bitfinex to exchange between the two.