Another way to think of it. A penguin gets on just fine, until you put it in a desert.
Same with genetic intelligence, if you put it in an environment where it will thrive, it will. If you don't, it won't. If someone who is incredibly intelligent ends up being mowgli in the jungle book they probably won't crack the Riemann hypothesis. Likewise if you put someone who's less intelligent, pay for them to go to the best schools and have the best tutors and go to university. Admittedly they'd probably do better than if they didn't have those things, but they wouldn't be able to do solve the Riemann hypothesis either.
I think the idea that some people are born smarter than others is politically inconvenient to the equality brigade, because being smart has a definite advantage in the society we live in today. There are some people who truly believe if they were swapped as a baby with Einstein, they would end up being as monumental as he was because all of his intelligence is due to his upbringing and environment. It's laughable.