> Their assets equal liabilities

This is irrelevant, though. You can commit fraud without losing any money - Martin Shkreli was famously convicted of fraud a few months ago even though the victims all came out ahead.

In the USDT -> BTC -> USD trip, there is a point, namely the BTC part, where more USDT has been issued than USD has been accepted. If they claim USDT is backed by USD, but there were points where that was not true, they have committed fraud, regardless of whether they were lucky enough to come out ahead.