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.
Yes, that would be the origin of it, I think you’re right. And there must have been a reason she was asked about Mombasa.