You're just being pedantic now. We make decisions every day all on imperfect information. There are enough red flags around Tether that I wouldn't bet any real money on it or on the things its propping up.
The burden of proof is on Tether. They're the one making the claim they're fully backed.
The point is that absence of evidence is evidence of absence - in this case, rather strong evidence of absence. Without perfect information, of course nobody knows - but most people do in fact know with sufficient certainty to not bet on Tether (and to advise people similarly not to). Merely "not being certain" of something does not, in fact, mean that it's unclear what to do.
If I tell you that I am a Nigerian Prince and that I definitely have a billion dollars somewhere and that you need to just send me a small sum of 10k to release that money, would you not tell people that it's an obvious scam?
I mean you don't know for a fact that I am NOT a prince right? And if you ask me for any documents I'll just say they got lost or it's too difficult to produce them.
In this case the absence of information is the proof that you need to say that tether is a scam.
(Downvoted for apparently wilful failure of reading comprehension. "Certain enough to advise [caution]" obviously doesn't mean what you are using "certain" to mean.)
… yes? I can be full enough that I don't want to eat any more rice, but not so full that I can't squeeze in some pudding. The word "enough" is right there in front of you, indicating that the adjective it modifies has a quantity. If I am full, I don't want any more food; if I am full enough not to want X, that doesn't mean there is no food I will eat.
As for Tether I'm certain enough to advise anybody investing it or any currency supported by it to not invest money they can't afford to lose.