Obviously I can't read the author's mind but my interpretation was not that she was bothered by not having other black women around her so much as she was by the consequences of that lack of diversity. She felt left out and passed over at times. She felt like she had to constantly defend her opinions, her self.
I don't think diversity in the work place is something that is an absolute must but I think in cases where that lack creates a hostile work place for some employees there is absolutely an issue. If you're co-workers are making racist comments/assumptions about you and your manager's solution is to ship you off to another office, there is absolutely a problem.