It's in the book Maraniss wrote iirc, seems the kenyan told an acquaintance at the East West Centre how disappointed he was that Kezia had a child to another man while he was away. That alone, might just be self-serving, however, you have 'granny' Sarah telling some of the children of Kezia (in 'Dreams')
none of you are his children which Auma, in the book, then explains that the old lady had a hard life and she's just jealous...
All that might mean nothing, but when you take into account that Kezia herself has stated she worked as a waitress in Mombasa, it begins to come together...why there might have been an Obama birth in Mombasa in 1961, (iirc, it was in October) and why, if Hillary did search the UK archives as has been suggested, she might have been disappointed.