Any complex adaptation, requiring many genes to work together, cannot all evolve at once, it would be too unlikely a mutation. Instead, pieces evolve one by one, each individually useful in the context they first appear. However, there is not enough selection pressure to evolve a new piece unless the old pieces are already universal, so you would not expect anything complicated to exist in some but not all members of a species.

I get that. I don't see how that could imply that quantitative variation must be controlled by a single gene.

I also don't see how the magnitude of variation in intelligence affects the argument ("particularly wide intelligence spread" is subjective).