The most logical answer is Barack Obama himself provided that information directly to his agent. The document was created before the internet so no one would have found much biographical information on this nobody from any independent sources.
Obama could actually have been born in Kenya or he may have made the story up to enhance his background, thinking the lie would never be found out. Either way, the onus was on Obama to explain this document but the media were hopelessly compromised and the republicans too terrified of being tagged racist.
More likely the latter - since it is a more interesting and useful backstory than being fathered out of wedlock by Frank Marshall Davis - but using such a fable to further his career is disqualifying in itself.