So we learn at the end that Sisko’s birth was engineered by the Prophets, right? That he was always destined to be the Emissary because that’s how it had to be, from the nonlinear perspective of the Prophets.
So what if they based him on a man from Earth centuries before (perhaps one of his father’s ancestors)? He does say at one point that maybe God is trying to tell him to quit writing and go into the restaurant business, betraying a love for cooking, which maybe he passed on to his children - and maybe he passed this on to his descendants, one of whom moved to New Orleans and opened a Creole kitchen, which would stay in the family for many generations…
They chose this man because by some quirk he had genuine future-sight and saw forward into the life of Sisko because of their connection established by the Prophets - creating a self-sustaining loop.
Now of course that doesn’t explain why all of the people in Benny’s life are so similar to Sisko’s people (or is it the other way around?), but maybe there’s something there about celestial-temporal archetypes, or Benny is projecting those faces and personalities onto his coworkers because of his strange and exceptional mind.
I always took it as Benny being a dream, someone informed by Sisko, the Prophets, and the Pah-Wraiths.
I think it is important that the Prophets picked someone who was very aware of humanity’s issues with discrimination and could rely on knowing the lessons learned from humanity to help Bajor transition.
Strange New Worlds confirmed that Benny Russell was a real author in the history of the Trek universe, though. So he was definitely a real person, though perhaps the version of him connected to Sisko was from some other reality.