If that quantity of Tether (1/75 of the float) were dumped into the open market for USD, you'd expect to see the USD change rate fall. That clearly hasn't happened. The rate has been very close to 1:1 forever.
There's this unsubstantiated conspiracy theory that as I gather goes like this: Tether prints tokens, uses them to buy bitcoin when the USDBTC exchange rate falls. These Tether issuance spikes always happen after BTC corrections, and this is taken as evidence of pumping USDBTC.
The problem is that if there were legitimate buying of BTC with Tether, which is plausible after the ass-whooping USDBTC has taken recently, new Tether would need to be issued to keep the 1:1 peg.
Sources critical of Tether IMO don't provide the evidence needed to distinguish the two scenarios.