Apparently the military ate GPs cognitive abilities through it's brainwashing in bit camp if he believes you can't have valid votes through mail. You sign affidavits claiming you are who you are in most states. Some states everyone gets one. Then they mail you the ballot. You sign another affidavit when mailing it back in. It has to match the info and signature you sent originally. So it's not like you can get multiples, or vote in multiple precincts. Kind of like in person voting. It very, very difficult.
I know he'll likely point to the 17% or so of ballots rejected as proof, but most were rejected due to being post marked after the election date and/or received late. Signatures missing, misspelled names a d addresses. You know the same shit that get people turned away from in person polls.
But yes at this point, it's blatant fear mongering and conspiracy theory.