He is way, way off the mark here. In Virginia, to name one place, there were laws on the books permitting whites to kill African slaves. Blacks. In America, it was white enslaving black, and the laws show that. What was meant by "negro", if not blacks? Puhlease.
Huck: He is way, way off the mark here. In the facts you point out I do not see any contradiction with the author's statement.
Consider a school, where a number of students' parents advance an idea that males teach mathematics badly. They point to the facts: numerous misstatements of well-known theorems, poor explanations in class, etc. As a consequence, these parents are sick and tired of "men teaching mathematics."
You probably would say exactly the same thing the author does about slavery: it is a "gross misconception" that poor knowledge of your kids is about men teaching mathematics. You reply by saying "way off the mark here:" in this school, all the teachers are male (as you put it, "In America, it was white enslaving black.")
You can see the point: one cannot use such a small sample and conclude something about gender and teaching.
The author makes a similar point. When looking a wide and complex social phenomenon such as slavery, one should not "localize" it to a particular area. Slavery was not, as he correctly states, about whites and blacks: it was about power. Those who could enslaved the ones who could not defend themselves. In most cases, the owners of black slaves were black; it is they who sold their "possessions" to others --- other black Africans, Arabs, Berbers, Europeans, and American colonists.
Do I need to add that slavery disappeared from Europe only recently and not yet from Africa? The Roma people (Gypsies) have been enslaved for centuries in Romania and freed later than the American slaves.
Of course, the local aspects affect the outcome. There is no doubt that racism in America has a lot to do with the fact that slaves happened to be black. Much the same way the West-Europeans looked down on Slavs who were routinely sold to them into slavery (as you probably know, the very word "slave" got into Latin after a large number of slaves from Eastern Europe ended up in Spain around XI century). You are correct, of course, that various discriminatory laws were stated in terms of the "Negro race."
Yet to say that "slavery was about white people enslaving black people" is a misconception. This is less than obvious only because we teach very little history and critical skills in our schools and because the leftist, power-hungry Black leadership promulgates this false image. "The Roots" contributed to that process.