Bitfinex is conflating Tether (USD₮) and USD: “Bitfinex lets you receive USD₮ from a Bitcoin blockchain address and treat them as a USD deposit. Similarly, you can withdraw funds to a Bitcoin blockchain address as USD₮.” [1]

Whether they are holding a lot of USD₮ or not, I don't know. But since they cannot receive USD, by construction their USD holdings are limited.

[1] https://support.bitfinex.com/hc/en-us/articles/213919369-Wha...