His claim wasn't that there will be some amount of fraud... it was that they won't be "anything less than substantially fraudulent" (https://twitter.com/realDonaldTrump/status/12652558351245393...).

Claiming mail-in votes will be majority fraudulent, and by implication that the entire vote is invalid... is a much stronger claim, which IMO requires much stronger proof.

Given that mail-in ballots have been in used for a long time, there's a good history of data, so it's not predicting the future out of nothing, but based on past evidence.

The twitter fact-check link in fact goes into that precise thing.