In any case, fifth season is not parellel to real history nor contains stereotypes. The author goes out of her way to give distinct personality to each character and no group is composed of same-like individuals. All biological groups have members of differing opinions and personalities. Shared traits are carefully explained as result of situation.
I don't know what you mean to say by that description of orogenes and fulcrum. Black could not blendin (except few very white individuals at risk to have dark child), they were not separated to largely self-governing institution). Instead they lived in households and plantages. Blacks indeed had own dialect, but so did any group of people separated by livestyle or rules. Different classes and separated places always grow to differ in language and habits. None of it is specific to black history in United States or very closely parallel to it.
It shows that Jemisin do know how groups of people develop cultur and shows her opinion on race being social construct. It does not show special closeness to black history in USA except in some taking cues of how people behave. She is educate and knows psychology and likely sociology well.
I think that you read too much from wordplay+callouta and ignore massive massive differences between orogenes and real world blacks - both during and after slavery.
"Technically it makes no sense to call this a race at all; they emerge from every people of the Stillness in roughly equal proportions, though more of them live to adulthood in the Arctics and the Equatorials because the Fulcrums are located in those regions. There’s nothing visually distinct about orogenes; the thing that makes them unique is perceptual, behavioral. Yet race in our own world is a social construct, not anything related to actual biology, so it makes sense that a world which has such complicated feelings about orogenes would conceptually fission them off from the rest of humanity. In fact, as Essun alludes at one point in TFS, Old Sanze officially classified orogenes as non-human a few centuries back. The better to oppress.
But there’s also something about the common experiences of orogenes which lends some substance to the label. Orogenes of the Fulcrum use a highly specialized vocabulary — e.g. torus, crucible, grit. They aren’t used to hiding, which is one of the reasons it’s rare for them to successfully escape; having spent years among their own kind, there are markers in their behavior which make them stick out like sore thumbs among any non-Fulcrum population. Only those who lived outside the Fulcrum as children have much real hope of blending in. But orogenes who grow up outside the Fulcrum — ferals, as Fulcrum orogenes call them — have their own unique traits. They share the markers of an oppressed group in hiding — and just as black people can sometimes spot those who are passing via “blackdar”, and just as other hapas often know on sight that (frex) Keanu Reeves is One Of Them, it’s something that can be seen. It might be subtle, protective camouflage… but it’s visible to those who have reason to look for it."
(http://nkjemisin.com/2015/08/creating-races/)
So I'd say that it's intentionally closer than you believe it is. But also - sure, it's different, but then isn't the book being discussed here also different?
Also, regarding orogene/rogga - it's not just the second part. Look at the first word closely, and consider the letters it consists of. Or just read it backwards.