In my opinion, best case scenario: Tether is buying Bitcoin and other assets with Tether, and has been able to sell them at a profit as prices rise, leaving their reserves at or above 100%. This sort of initially unbacked issuance may be a crime in and of itself, but its possible Tether holders would have a legitimate claim on any assets if Tether fails or is shutdown and wouldnt take a loss (though it could take years to get back their money).
Worst case scenario: its entirely, or almost entirely fraudulent, and Tether insiders have embezzled and spent as much of Tethers available hard currency as they can.