As I said in the article, I agree that not all African kingdoms were alike. However, I doubt there are any voluntary kingdoms--people don't choose to be exploited by a few powerful people when they could have a much freer arrangement. So while I appreciate your addition, and particularly the mention of Mulambwa, I have to say your comment about "true African kingdoms" is a "no true Scotsman."

Do you have any source for your belief that slavery was nice before the Arab traders provided a major market?

And even if that's true, it does not excuse African slavers. No one forced them to sell other Africans. They did that out of greed.