It is crazy to see how well he knows his base and how to get them to rally close to an election. Making them think everything is a liberal bias against them, and if they don't vote for his big government agenda they will receive a big government agenda. This is just one more way for him to get his base to believe everything he says versus people who actually prove what he says is a lie. He wants state run media and social media just like China. As much as he talks about hating China he would love to be China.
And this is specifically on a tweet calling the validity of the election into question. It's blatantly wrong, but he needs his base to believe him when he says the election is rigged.
What was blatantly wrong about it?
Mail-in ballots don't generally increase the instances of election fraud. All of the "what ifs" have been investigated in the states that have had general-populace mail-in for years and been found not to occur in either (a) numbers that sway the election or (b) numbers distinguishable from in-person fraud (which is usually of the form "person not eligible to vote, but voting office screwed up and granted them a card" or "person moved and failed to notify election boards of the relocation; voted in the wrong district").