Here's their reserve breakdown as reported by Coindesk,
- Cash & Cash Equivalents & Short-term deposits & Commercial paper: 75.85%
- Secured Loans: 12.55%
- Corporate Bonds, Funds & Precious metals: 9.96%
- Other investments: 1.64%
Up until today, I also have doubts about their true backing and their actual assets.
I guess, we won't know the absolute truth coming from them as they can cover up with the help of other third party companies.
So for me, it is better to hold your coin in BTC rather than in USDT. Just remember the 2019 admittance that they are not purely backed by USD assets.