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...

Bitfinex's Tether "holdings" are unverifiable and irrelevant anyway. What matters is what you can deposit and withdraw from their markets. It's not USD, it's Tether, and that is verifiable by anyone with a Bitfinex account.