I haven't read the book and don't even know the name of the author, but my guess is that they were neither practicing nor advocating for leashing people with dementia, whether black or not. So the question is, why is there a backlash against this as depicted in fiction? Fiction has depicted, and will continue to depict, all kinds of atrocious things. Where is the line drawn between what things can and cannot be expressed, and who makes such decisions?
And more so, what do people want to come from their outrage? It is one thing to say, "Man, Dances with Wolves was such a white person's view on Native Americans," quite another to act for its removal, or that all future films be vetted through some kind of filtering process that disallows people from making the films they want to make. I personally would prefer to allow such films to be made, if only so that we can have the conversations.
Furthermore, there is a place for art that makes us feel uncomfortable. Taking offense generally involves personalizing it, thinking it is about oneself, when in the end it can only ever be about the artist themselves. Or as the quote goes, in paraphrase, "non-fiction is about the world/others, fiction is about ourselves" (can't remember who said it). Meaning, a work of fiction is entirely about the author - every character is an aspect of the author, every idea the author's version of it. It is a peak into the mind of the author, and we read it through our own minds, our own experience.
As for writing from the perspective of the "other," this is at the core of the matter and what Yossman discusses. Everyone is other, regardless of ethnicity, gender, or other intersectional categories. And of course, orcs and other fantasy creatures are especially other to us all - as are fantasy creations inspired by real world cultures and ideas. So while at the same time, everyone is other, everyone in a work of fiction is not--it is ourselves.
It is one thing to consult someone about depicting Jews (if non-Jewish), LGBTs (if non-LGBT), blacks (if not black), etc. It is quite another to consult someone about one's fantasy world. Even in the case of a "fantasy Africa," it is still a fantasy setting...a fantasy Africa could and almost certainly would have real and perhaps even massive differences from our own world. For instance, maybe it never experienced European colonization. That would change just about everything. Maybe its people have entirely different cultural assumptions than the Africa of our world, yet only share the trait of higher melanin.