So that makes Mombasa a particularly unlikely place to pick if one were making up a Kenyan birth story—rather than dealing with actual facts.
Actually, for some reason Mombasa was the city mentioned by the interviewer (can't remember his name) in that recorded interview with Obama's grandmother. (In the full audio, she disputes that he was born in Kenya). Why Mombasa was brought up in that interview, I don't know (If anyone knows the full back-story, please let me know), but IF this document is a forgery, that would seem to be where the idea for Mombasa could have come from.