These population charts do not account for any future demographic changes. They simply assume trends will stay the exact same. While good for visualizing, it's far from being set in stone
Africa, especially West Africa, is getting wealthier and the wealthier your country is the less children your people have
It's why Latin American birthrates are lower than African birthrates and subsequently European birth rates are lower than Latin American birthrates
In the not-so-distant future countries like Nigeria will have the same demographic problems and lack of children the rest of us are having. The main difference is that for them it's delayed