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.
[2] https://www.nbclosangeles.com/news/politics/more-than-80-bal...
[3] https://www.sacbee.com/news/politics-government/capitol-aler...