Do you have a link for this? All I could find is an "attestation" of reserves from a few months ago but could not find any evidence of an audit by a third party that might constitute "proof".
Latest report from an independent auditor: https://assets.ctfassets.net/vyse88cgwfbl/63oJePOHqIvrcnXWMP...
Key stats: "Cash & Cash Equivalent & Other Short-Term Deposits" are $73,567,577,334 (out of total assets of $86,499,251,218). Remaining assets are precious metals ($3,270,216,824), bitcoin ($1,676,229,360), corporate bonds ($115,496,758) etc.
It is no rocket science to see why USDT is in black now - with 83B USDT, they have that much cash available to park in T-bills and earn 5+% interest just like that. Even if they were in a hole a few years back, they would have recovered over the last 12-18 months when interest rates started rising.
Rising interest rates certainly helped them and USDC/USDT are a money making machine now for their owners.