The phrase does indeed say something. "anything less than substantially fraudulent" means near and total fraud. Claiming ~100% of mail-in ballots are fraudulent is baseless and untrue.
I just don't see how you can go from "anything less than substantially" to 100%. You can't go from "substantially" to any percentage.
For an example, I could pull the same trick on your comment. I could try and convince people that "does indeed," implies 100% confidence, and then I could point out that since from a Bayesian perspective you should never reach 100% confidence, you can't know what you're talking about. Obviously nobody would buy that, because it's not you that wrote down 100%, it was me.
You're getting entangled in the details. It's clear from his tweets that what he means is that mail-in ballots will cause a rigged election. We shouldn't trust rigged elections, so he's saying that if the election includes mail-in ballots, it will be untrustworthy.
In order to make that reasoning (you could say that arguments are not false or true, they're just arguments) he's using false (CA will not be sending ballots to "anyone") and unsubstantiated (mail-in ballots have been working for years without extended robberies and forging, why would this be any different?) claims.
You can search and reason about the words so that "technically" he isn't lying. But the actual message he's sending is very, very clear.
What does 'substantial' mean? "of considerable importance, size, or worth.". We further constrain the definition with 'anything less than'. So what we're saying is, 'of an especially considerable importance, size of worth'. Okay, well, what does that mean? It has to be a number large enough to influence the election result, but there's no evidence election results have flipped elections due to enough fraudulent mail-in ballots. With this interpretation, the numeric quantity does not matter, but the impact does, yet it is still untrue.
