I think we all know the money isn't there. With luck, some investments have paid off, and perhaps they can pull a rabbit out of a hat and actually have the necessary collateral. But I think any major drop in equities or bonds will start to expose what's really there and what isn't.
What I don't understand is why exchanges are willing to take it. Everyone says that most people only have USDT for minutes, while they are doing a transaction, so their risk is very small. The chances of it blowing up in any one hour time frame are really small. But its perhaps the exchanges that need to hold it so that customers can transact back and forth. If there is a ever a so called "bank run", everyone will want out of USDT, and its exchanges that will be holding it all. Then when they go back to the issuing company to exchange of USD, good luck.
So you would think that they would do their due diligence, but its perhaps just the cost of doing business. If each exchange holds just 1 or 2 billion of this stuff, then its a loss they can absorb. Not transacting in it might cost them lots of lost revenue.