I've never been sure why racists love this argument, as though the participation of Africans in the European system of chattel slavery somehow absolves the creators and beneficiaries of the system.
The African slave-traders benefited too. They were sold for European goods. I'm not sure why people get defensive when people point out this fact, except if they want to just blame whitey for everything and keep holding up Africans up as the noble victims. Sub-Saharan Africans nations at the time were just as monsterous as European nations. Because they're human beings.
North American indigenous peoples had their own violent conquering empires, like the Aztecs and Incas, who murdered, plundered,
****, and enslaved other indigenous groups. They would sacrifice conquered people by ripping their hearts of their chests alive. That's how terrible people were back then. There were inadequate or no human rights, there were no international rules of war, it was a war of all against all, we behaved like animals. European nations were all busy murdering and raping and conquering each other too.