So Tether is doing something akin to fractional reserve banking? From what I understand, this shouldn’t cause a problem unless there is a massive run (everyone trying to convert their USDT into dollars at the same time). Since most of the USDT is owned by big exchanges who need it to provide liquidity and have no interest in crashing the crypto market, I don’t think this is likely to happen.
There isn't any real visibility into the balance sheet, but I'd guess most of the "collateral" boils down to margin loans issued at the exchanges, and since the house has the edge on those platforms, their ship stays afloat.