Well… “unsubstantiated” is a bit ambiguous, but in my understanding of prevalent usage, it means “not substantiated”, as in, so far there isn’t evidence, as opposed to “false”, as in, “the evidence shows the claim to be untrue”. It means “not yet known to be true”, not “known to be false”.
If he wanted to be unequivocal he could have said “false”.
DeSantis is trying to position himself as a more reliable, rational character than Trump, for those who don’t like Trump’s erratic qualities, yet to the right on policy and just as bombastic and willing to pick a fight over culture war imaginings and phantom issues. This is bound to fail. There is no path to electability through that combination.
It may be a Hail Mary to be the backup candidate, but I doubt that works and if it does, he loses to Biden anyway.