Exactly - I suspect there are internal checks and procedures to prevent scams involving overpayment and immediate refund.

Or just as simple as “everything has to settle before we send a check” so you can’t use the US Treasury as part of a check cashing scam.

Correct. He’ll get the refund the day after a clerk reviews the transactions and clicks the refund button. The “Please allow N–M weeks…” verbiage you find in many types of contracts is just the other party giving themselves time to conduct that manual review. It should never be taken as an estimate of how long the other party thinks it will actually take.

Earning interest from whom? It's the US treasury, in the end they are the one that create money out of thin air. They don't need to swindle a few bucks here and there to invest in short term schemes.