What's the difference? Is an attestation at least better than what USDT provides?

I am not an expert, I just looked it up. If I understand it correctly, an attestation is just someone witnessing a legal act like the signing of a contract and testifying that the parties signing the contract are the ones named in the contract and that nobody was forced to sign it with a gun to his head and such.

If this understanding is correct, then the attestations may just say that the company behind USDC prepared and signed those statements about their balances, but may say nothing about the correctness of those statements as they did not audit the company and therefore did not obtained independent evidence for the correctness of them.

I read one of the attestation and would read it as saying something stronger and closer to an audit but I am not an legal expert and find the language used quite hard to parse. Maybe someone more qualified can expand on this.

This is what they claim, in the March 2021 report:

"US Dollars held in custody accounts are the total balances in accounts held by the Company at federally insured US depository institutions and in approved investments on behalf of the USDC holders at the Report Date."

Note the "and in approved investments". Not cash.