It's likely that most/all of that stablecoin was amassed by selling other crypto, which is zero sum. It's extremely unlikely that people took fiat currencies, and bought USDT directly from Tether, which is what they claim is happening.
Totally irrelevant, the man's got a vested interest. Believe nothing short of an independent third-party audit from an auditor who doesn't walk out half way through like the last one did.
The reason I say this should be pretty obvious to anyone whose familiar with the inner working of a Ponzi scheme or other similar investment scam.
Bitfinex and Tether wanted to legitimize the use of Tethers, so they found one guy, Matuszewski, they knew was in a position to help them do so. They actually set aside a relatively tiny slice of capital (a few million out of 3 billion) and treated him the way they should be treating all customers but weren't. Then they let him loose on the talk circuits to tell the world of the sheer upstanding nature and legitimacy of Tether.
This is exactly the kind of stuff Madoff did.
Audit, or gtfo. (To Tether, Inc. not you of course)
No sane person would hold USDT over USD.