Yeah, that "slaves were better off than blacks in Africa" stuff is straight White Nationalist stuff.
And arguing that they lived better than SOME whites is even more absurd. Yeah, the ones that survived the Middle Passage, got separated from their families, survived beatings, were passive enough to not attempt to run away, and were lucky enough to live in a big, rich plantation with an extremely generous master may have better living conditions than some of the poorest whites.
But human freedom is THE most important value, so nothing else matters. Ask those poor whites if they'd rather tough it out free or become property for the rest of their lives and their children's lives and their children's lives. It's a complete no-brainer.
But again, this is from the liberal perspective upon which Western civilization has been built on since the Enlightenment. Not everyone is going to agree with it.