I am sure I do lack lots of perspective on this as I am not even a Kanye fan at all. However, my first impression is that he didn't mean slavery was a choice literally in the historical record. If he does, just point anyone to a basic history class and this can be a refresher course on basic American history, and how it absolutely was not a choice at all, and a war was fought to break the institution of slavery.
I am assuming he is making a political point with some kind of exaggerated, artistic flourish on it. If he's going republican now, and buying into all their ideology about individualism and pulling yourself up by your bootstraps, and telling people to not automatically vote democrat - it seems to me like the point he is trying to make is he thinks black people are too complacent politically, and he's conflating voting for democrats and conformity of thought with voluntary slavery to make his point in a superrrrrrrrr exaggerated way. So he's just another guy who decided to back Trump to go for "change," and is either 1) completely misinformed about Trump's policies, or 2) is a legit republican.
Is it an offensive exaggeration? Yes, I can see how that is deeply offensive. I just didn't think anyone was taking it at face value, since its so easy to prove slavery was not a choice.