He wasn’t talking about absentee ballots.
And how are absentee ballots sent if not through the mail? No state allows ballots to be emailed for obvious security reasons. Why should one form of voting by mail be more susceptible to fraud than another when both require a signature on the envelope that matches the one on record and a valid driver's license or SSN to register? The only difference is in absentee voting you have to explain what your health issues are. I don't see how that prevents any voter fraud.
You’re looking for a problem. Look elsewhere