Before I clicked on the link, I assumed this was yet another egregious lie. But this one really doesn't qualify. It falls squarely in the category of reasonable opinions. Let's look at the claim: "There is NO WAY (ZERO!) that Mail-In Ballots will be anything less than substantially fraudulent". Clearly the veracity of the statement rests on the word "substantially." It's inconceivable that no one ever takes, for example, an elderly relative's mail in ballot and votes it fraudulently. So for this to be a reasonable opinion, it merely has to be reasonably possible for the claim to be true.
So, let's consider if such fraud could possibly be substantial. Let's use California as an example. Registering to vote is easy, all you need is a web browser[1] and a mailing address where you reside or with a resident willing to give you ballots or ignorant that they are receiving them[2]. Also, the State of California faces an impractically large legal hurdle to reject suspected fraudulent ballots[3] so it can be assumed that most if not all will be accepted. It seems clear based on the observable facts that substantial fraud is certainly possible under the California regime. Please note I am not claiming such fraud actually happens, merely that it is easily practicable for an organization as well organized as, say, a political party. To be honest, I bet literally hundreds if not thousands of readers on this site could build "California vote fraud as as service" as a side gig. Let's disrupt the electoral process for a billion dollar valuation!
From this I conclude that while I personally disagree with the President, this particular statement is a (probably, I sure hope) incorrect opinion, not a factually incorrect statement.
[1] https://covr.sos.ca.gov/
[2] https://www.nbclosangeles.com/news/politics/more-than-80-bal...
[3] https://www.sacbee.com/news/politics-government/capitol-aler...