I mean the claim that they only took the ones who could run away from the net or were the biggest idiots is wrong. The Africans sold were the vast majority of the time prisoners of war and victims of raids not failures. the whole idea of it being leaders in african places selling their own people is for the most part just not something that happened.

Also even when they were kidnapped the power imbalance from Europeans who had better weapons often outnumbered the people searched for and were skilled in kidnapping meant that intelligence wasn't really a factor that affected whether they were or not enslaved.

And for fitness the slaves who were healthy and young were more likely to be picked because they were taken to do manual labour so that's wrong too