He has.

He talks about it at the start. If your body and brain in development does not receive enough nutrients, it does not fully develop.

As a volunteer teacher I have seen African kids disconnect after just 2 hours of class because the food they ate was weak in nutrients.

This usually happens with iodine. If the pregnant mother has not enough of it, the baby will develop with smaller IQ.

The same happened in Africa in sport for a long time. It could not compete with the USA, even when most of them actually were genetically from Africa. In the US, they could develop fully.

You're conflating phenotypic variation and IQ potential.

The linked article starts with

"We were born in a wealthy country to white, well-off parents who lived in thriving communities and were able to send us to excellent schools. These factors, among many others, put us in a great position to be successful."

It would be more intellectually honest if Bill Gates had mentioned his very high IQ as a factor in his success.

This article estimates an average IQ of 82 rather than below 70 for sub-Saharan Africans.

https://www.sciencedirect.com/science/article/abs/pii/S01602... Intelligence Volume 38, Issue 1, January–February 2010, Pages 1-20 A systematic literature review of the average IQ of sub-Saharan Africans M.Wicherts, Conor V.Dolan, Han L.J. van der Maas

A response by Lynn and Meisenberg in the same journal, defending the below-70 estimate, is at https://www.sciencedirect.com/science/article/abs/pii/S01602... .