Earlier quoted context omitted.
"Natives did not make good slaves, and so Africans were imported." That alone is so innocuous as to appear in elementary school textbooks. Is this really what the whole row is about?
No, it's because he said it was due to genetic differences between population groups. This makes him a "true" racist in that he believe there are different races, or sub-species, of homo sapiens (sapiens). There is a scientific consensus that this is not the case, because races only exist if population groups are completely isolated, which humans never have been. https://en.wikipedia.org/wiki/Scientific_racism
Use your own brain. Denying the existence of races is completely absurd. See for instance:
http://infoproc.blogspot.com/2008/01/no-scientific-basis-for... and https://jaymans.wordpress.com/jaymans-race-inheritance-and-i...
And lots more reading here: https://jaymans.wordpress.com/hbd-fundamentals/#race