Their website used to say "Our reserve holdings are published daily and subject to frequent professional audits", for years. This was a lie, intentional deception for financial benefit. They're still promising they'll get around to it eventually (https://www.pymnts.com/cryptocurrency/2022/tether-audit-prom...).
> In the face of persistent questions about whether the company actually held sufficient funds, Tether published a self-proclaimed ‘verification’ of its cash reserves, in 2017, that it characterized as “a good faith effort on our behalf to provide an interim analysis of our cash position.” In reality, however, the cash ostensibly backing tethers had only been placed in Tether’s account as of the very morning of the company’s ‘verification.’
> On November 1, 2018, Tether publicized another self-proclaimed ‘verification’ of its cash reserve; this time at Deltec Bank & Trust Ltd. of the Bahamas. The announcement linked to a letter dated November 1, 2018, which stated that tethers were fully backed by cash, at one dollar for every one tether. However, the very next day, on November 2, 2018, Tether began to transfer funds out of its account, ultimately moving hundreds of millions of dollars from Tether’s bank accounts to Bitfinex’s accounts. And so, as of November 2, 2018 — one day after their latest ‘verification’ — tethers were again no longer backed one-to-one by U.S. dollars in a Tether bank account.
No, they’re not.