Update: I talked to someone at IRS. The 4464C letter is merely to extend their deadline from the CP05 letter, and there is no issue at all with my return, just that they admit they are slow, and blame it on Covid, also despite what the 4464C letter says about 60 days, it will probably be 120 days before I get my refund. So they issue successive letters to taxpayers, vaguely implying blame on my part, in order to justify missing one deadline after another.