It's possible that they happened to encounter some exceptionally intelligent mathematicians who never managed to produce exceptional mathematics. On the other hand, it would be worth asking how many (if any) great mathematicians had high but non-exceptional intelligence.

Intelligence seems to account for roughly 40% of the variance in the logarithms of mathematicians' research productivity, with the remainder accounted for by other innate abilities and environmental factors. This is consistent with most exceptionally intelligent mathematicians producing unremarkable math, and also (given the rarity of people with exceptional intelligence) consistent with some great mathematicians not being exceptionally intelligent. I'll write more about this later.