The message isn't clearly false. See this article for example;Seriously. I am getting a "we have always been at war with East Asia" vibe from this latest uproar.
If you use Google search tool to look up "mail-in voting fraud" and limit the search to before April 1st, you get a lot of concerned articles from NPR, NY Times, Propublica, etc, that mail-in voting fraud is a problem to worry about, and that expanding mail-in voting might lead to more fraud (and they also think Republicans will benefit from this expansion): https://www.google.com/search?q=mail-in+voting+fraud&source=...
But then Trump tweets about and there is a 180 and now it is disinformation to claim that a massive increase in mail-in voting will lead to a massive fraud problems.
Two old quotes are interesting to me:
From NY Times in 2012:
> “Absentee voting is to voting in person,” Judge Richard A. Posner of the United States Court of Appeals for the Seventh Circuit has written, “as a take-home exam is to a proctored one.”
From Pro Publica in March 2020 ( https://www.propublica.org/article/voting-by-mail-would-redu... ):
> “To move from a couple of thousand to a couple of million requires an entirely different infrastructure,” said Tammy Patrick, a former county election official who is now a senior adviser at the nonprofit Democracy Fund in Washington, D.C.
Just from those two quotes, it is not at all unreasonable to extrapolate and predict that massively increasing mail-in voting on a tight schedule is going to be a huge fricking problem. I don't know what the answer is, and I don't which party is going to benefit more. And even if you think Trump is wrong, he is still making a prediction that is based on real concerns, which is something that politicians do all the time, it is not a blatant error of fact.