It all depends on what you mean by "diverse" really. In some circles, it seems to be a code word for "not white people" as if white people are all the same.
If Africa was indeed used as an example of high cultural diversity, and we agree it's mostly "non white people", then I don't think that would be an issue here, right?
From what I read it was something along the lines of it lacks multiculturalism and is closed off.
Of course that isn't the issue where an entire continent is compared to a small American town...
Edit: Oh I see, of course I have never visited Oakland so I do not understand what is said between the lines. Unless the implication was a small town is as diverse as a continent in some kind of hyperbole.