The math in the article is wrong because it’s starting point is wrong. The number of possible illegal aliens in Florida is not the sum of how many Homeland Security thought were there PLUS how many Pew tallied in their survey, but some other number, and at a minimum maybe an average of the two, which would be 1,125,000.
That more likely starting point makes the rest of the math null and void, as an argument, because it does not lead to a big enough difference in the voting outcome.
It says in the article that the Homeland numbers were legal immigrants and Pew numbers were illegal immigrants.