I don't read that in their comment? They said genocide has a specific definition, and the Atlantic slave trade does not match it. You are free to disagree on that point. I personally don't think it's a very good point to make here.

Instead you made some argument that they are trying to argue that it wasn't that bad, which is not based on a literal interpretation of their comment.

Can you believe white supremacists use the same kind of thinking? They also toss around the word genocide where it does not apply, assuming one against whites. If you were to say what the commenter said, they'd do the same: Instead of engaging the arguments based on merit, they'd accuse you of wrongthink.

Better to let people be honest and judge argument's on their merit rather than start accusing people of wrongthink so easily. Perhaps this is a pretty big problem in the politics right now.

If it becomes impossible to correct the definition of words without being branded as a wrongthinker, then what good is that going to do?