People always refer to Tether 'printing money out of thin air', but they never consider the prospect that investors are wiring money to exchanges, having Tether printed for their use, and then using that USDT to purchase cryptocurrencies.
Bitfinex is huge. Bitfinex also has an OTC counter, which has been known to broker trades in excess of 10-50M USD. USDT is often the medium for which these trades are executed, so it makes sense that a lot of Tether needs to be printed when BTC is crashing, as that's when the larger players decide to buy in the most.
There is a lack of transparency, and I wouldn't be surprised if things don't look perfect behind the scenes, but that doesn't mean the entire thing is a billion dollar scam conspiracy that the largest exchanges are all in on. Not all transparency is in the users' advantage in this area either. Do you know why Bitfinex doesn't tell you exactly where their servers and wallets are located? Probably because they hold over a billion USD in BTC in a single address.
Keep in mind that the market clearly values Tether at around $1USD, almost always. That's a good sign that the market as a whole does not think Tether is going to suddenly collapse and take the entire ecosystem into it at any point in time.
It's important to read criticism like this and consider it, but not believe it blindly. There are a lot of feasible middleground scenarios between "is a complete fraud and scam" and the exact opposite.