"Borrowed" might be strong.
We don't know how strong the relationship between these companies is. One possibility is that Bitfinex wants to allow customers to immediately withdraw any USD deposits to other exchanges that accept Tether-USD. If that is the case, then effectively all deposits into Bitfinex are immediately "sold" to Tether in exchange for Tether-USD. That would mean that Bitfinex does not have any USD at all. Therefore the balance of USD in Tether's bank accounts would include all Bitfinex deposits.
Except for the opacity involved, nothing here would actually be misleading or obviously unethical; whether it is illegal is another question.