Americans can learn about slavery from an American perspective (without spending 3/4 of the lesson learning about the Bantu societies and cultures from which slaves were largely taken) while still acknowledging it as a stain on our nation's history.
I don't think anyone is suggesting that.
To my understanding, anyways, teaching history from the perspective of "marginalized groups" in this particular example would mean looking at slavery from the perspective of slaves. While it shouldn't be the
only perspective considered (slaveowners and abolitionists are of course
extremely important viewpoints to consider as well), I think it would be ... well, frankly bizarre to teach a course on American slavery without ever looking at primary sources created by slaves. Likewise, it would be an odd choice to talk about immigration in the late nineteenth century without considering the accounts of those immigrants. American history is more than just the history of white Anglo-Saxon Protestants.
(N.B. After all, depending on where you were in the South, black slaves
were the majority group or very near to it —it has always struck me as strange that "Southern heritage" almost always means
white Southern heritage when for most of US history, anywhere from a third to three-fifths the population of the South was black, depending on the state.)