Having now learned the terminology, assortative mating could indeed lead to speciation in the general case; it's just a special case of sexual selection. I find it highly unlikely that that will happen with human intelligence, though (despite the fact that it's been used in fiction more than once). The reason is that there's a limit to how big the variance can get- it doesn't grow forever, it grows until limited by other factors (i.e., there are only so many intelligence-promoting alleles that you can possibly have).

It would take intentional segregation to cut off all gene flow between the upper and lower halves of the intelligence Bell curve, thus allowing other differences to pile up between the populations, to cause speciation. And as noted elsethread, that's called eugenics and generally frowned upon in polite society.