Someone had a better suggestion in a comment chain above, which is that it's the responsibility of the dealer to request the rebate on behalf of the customer, and they apparently submit them in batches. So an even more plausible explanation is that it's a combination of people rushing to buy before the rebate expires, and the Tesla dealerships panicking to get all their paperwork for the last month or two submitted before the money ran out.

Either way it should be trivial for the government to verify this, and they should be doing regular audits anyways to make sure dealerships aren't scamming customers out of their rebates. Making a huge deal out of this now just seems like political grandstanding though, since there is no evidence of any wrongdoing.