There's a misconception that Star Trek depicts a post-racial society. There's no such thing and there never will be. What it depicts is a post-racist society. A character's heritage is still an important part of that person's identity and always will be something to be treasured. What it won't be in Star Trek's future is a handicap or divider. People won't resent the pride of others in their own culture, nor will they think that resources are unfairly distributed to others. As Jonathan Frakes likes to say Roddenberry told him, "There will be no hunger. There will be no greed. And all the children will know how to read." That's the difference.
And what Benny Russell wanted to say is that black people, or people of color at all, will still be around in the future he depicted in his story. Until that point, it was an open question if one would read mainstream science fiction stories of the period. It certainly was for Whoopi Goldberg when she saw Star Trek on television a decade later. People of color were simply invisible, especially in science fiction media. It would not be a far-fetched conclusion for consumers of that media to guess that they had been eradicated in an act of genocide by the periods depicted in futuristic science fiction.
Having any black characters at all would be a rejection of that idea. And having strong black characters with agency and humanity would not only challenge that, but challenge the contemporary ideas of racial hierarchy.
Remember, one of the other plots in the episode was that the magazine wanted to show pictures of the authors, but refused to show Benny Russell, KC Hunter, or Julius Eaton. They wanted to exploit the labor of women and people of color, but keep them invisible to their audience, and keep them in their place.
Representation matters on both sides of the typewriter, as it's important for underrepresented peoples to tell their stories as much as it is important for said peoples to be portrayed in those stories. As Star Trek has shown, and movies like Hidden Figures continues to show, is that when kids see people that look, talk, and act like themselves on screen doing amazing things, it shows them what might just be possible with love, and encouragement from others, and one's own tenacity.
That is why Benny Russell made Benjamin Lafayette Sisko black.