It's a long ongoing suspicion about USDT that they don't have 1:1 USD reserve. Especially when the books of the company was never audited. It's good that they came up with an explanation but they didn't provide any break up of the reserves yet. So it will be an ongoing dilemma until they provide a break up of their reserve.

Also I don't see any issue in using other assets as a reserve till the time they can justify the overall volume. But their books need to be audited by a reputed third party.