Well it does seem that a lot of the new USDT is going to Bitfinex, so it's certainly possible.

That said, it's certainly still speculation to suggest that they are using USDT to pump the price, and I can't see any hard evidence of this.

Given the fact that they were hacked in the past, they won't accept US customers due to regulations, and the possibility of this, it's probably best to just steer clear and keep safe.
Mt. Gox had a several times larger percentage of the total trading volume than Bitfinex.