The article is well-sourced, but the author's framing is very sensational, suggesting that Trump was threatening African Americans to learn the history of slavery or they'll end up back in it. I think from the video it's clear that he's simply referring to the larger concept that humans are at risk of repeating history if we don't learn from it. This article purposefully spins the worst interpretation of that to drive more clicks.

why are there concentration camps still in Europe for you to visit so that you learn from History and don't repeat it. Why are there statues to people who have done both memorable and not so memorable deeds so that you learn from their lives