There is certainly nothing that can be proven for or against about Tether unless there was a transparent audit of their accounts. Why will they not do this if they have nothing questionable?
Their general counsel has said in the past that the big auditing firms won't touch them:
“The bottom line is an audit cannot be obtained [...] The big four firms are anathema to that level of risk. We’ve gone for what we think is the next best thing.”
https://cointelegraph.com/news/tether-has-enough-usd-to-back-tokens-says-law-firm-in-unofficial-statementTether is pretty shady, which lends some credence to that statement.
Giancarlo, one of the founders of Bitfinex, infamously said a few years ago, "We're not criminals, but now we have to learn to bank like criminals." I don't doubt it!