So either they are lying through their teeth and shady as frack.
Or they actually don't have a bank account or even a few bank accounts with a big pile of USD and EUR to back their tether tokens - instead they have a whole bunch of sketchy promisary notes or something equally as intangible. And as you point out an audit would also have to try and establish that any simple deposits they have on hand aren't actually just money secured on loans. I'm pretty sure a corporation with the potential to mint billions in money could do all kinds of dodgy deals to magic $2B out of nowhere.

Rember those toxic mortgage-backed financial assets no one seemed to know where actually worse than junk and caused the 2008 crash... So when push comes to shove who knows if $2B in a bank account is really $2B in a bank account they can legally draw on, and if they did would they risk losing their balls to some shady as frack Russian maffia?