what makes you think so?
2. It's hard to imagine bitfinex draining 100% of the tether reserves and thinking it wouldn't blow up. So I'd hazard a guess that they took only a fraction, with the expectation of replacement before getting caught. (Isn't that the embezzler's cliché?) Since that fraction amounted to $850M, it's probably not a bad guess that they were fully backed or at least pretty close, so that Tether could continue any redemption operations.
The CFTC and DoJ were (and to all appearances still are, the latest news I can find on it is toward the end of 2018) conducting a wide ranging coordinated civil and criminal investigation of Bitfinex/Tether; there's no reason they would publicly release information before being prepared to take action, which wouldn't usually be when they first identify irregularities, but when they've chased down the responsibility and context issues enough to know the full range of charges they expect to file.
The idea that the NY AG not saying something about he results of an ongoing federal investigation in a state filing implies that the federal investigation immediately found everything to be just fine even though it continued for at least a year afterward and hasn't visibly ended is...not warranted.