It takes a very stable genius to think mailed-in ballots differ from mailed-in ballots.
I suspect it may be less foolish and more evil than that. They're dog-whistling that there are two different kinds of ballots. Ballots of people voting for Dictator Don, which are good and acceptable and totally legitimate. And ballots of people who aren't supporting Dictator Don, which are evil and fraudulent and totally shouldn't be counted.
Americans have a clear choice this fall, and for the foreseeable future. We can have a Republic, or we can have Republicans, but we can't have both. (Me, I'm for the United States and have nothing good to say about anyone who remains aligned with Trump and his fascists.)
He has got his cult to start parroting that voting absentee with a mail in ballot is different than voting by mail.