The market cap of USDC is something like $52 billion. No bank or banks would allow Circle to hold all that in a commercial bank account, so they diversified into holding treasuries. I don't really see the issue with that.
Then why not replace "US dollar denominated assets" with "cash and short-dated U.S. government obligations, consisting of U.S. Treasuries with maturities of 3 months or less" in the actual report and have Grant Thornton attest to that?
Their website says: "USDC is fully backed by cash and short-dated U.S. government obligations". I'm sure they'd absolutely love an official audit of that, but given the murky regulatory environment for crypto, I'm also sure that Grant Thornton is refusing to do anything more than it already is.
I admit I'm biased, I've used USDC personally & professionally, and I know people involved with running USDC. They're not evil geniuses. And even if they were, you could make so, so much money by breaking it ala Soros & the Bank of England, that I think it would've happened by now.
I'm sure Grant Thornton would be happy to do a full audit of Circle's assets IFF Circle was willing to hand over all of their banking/brokerage statements and allow them to contact those institutions. Verifying the quantity of cash and Treasuries sitting in an account, the length of time they have been there, and the counterparties to their largest transactions is trivial and doesn't compromise any trade secrets which is why it's such a huge red flag for Circle (and Tether) that they refuse to do this while continuing to tease that they will do it at some point in the indeterminate future.
You don't see an issue that USDC has not had a real audit for that $52 billion?
I could say my pencil is worth $52 billion and therefore I have the assets to cover $52 billion USDC and that would be the same as their 'transparency' reports which don't itemize a single asset, just an 'attestation' that the amount is covered.