Earlier quoted context omitted.
There's exceedingly strong evidence Tethers are not backed by USD - the Tether website claims "frequent professional audits", but they never completed their first one, were fired by their auditor, and cited a document explicitly stating it was "not intended to be, and should not be, used or relied upon" as proof of their reserves. It's a demonstrably fraudulent claim right there on their home page.
That is not strong evidence that they are not backed by USD. That is strong suspicion. > Lack of transparency does not necessarily indicate fraud https://blog.bitmex.com/tether/
Given the little we do know about the company the most reasonable assumption is that it is a slightly reworked Ponzi scheme.