It is irrelevant whether Obama attributes the utterance to Auma. In his book Obama is stating that his mother did not have any documents needed to prove who his father is.
Obama owns the narrative.
IMO, it is stretch to claim that by putting these words in his sister's mouth regarding Kezia and the situation in Kenya at the time, Obama is making an admission that back in HI there is no record showing his Sr. as his father, as the HI newspapers at a minimum indicate his mom and/or grandparents reported when they registered the birth (whether or not Barry was born there).
Ayers probably wrote this and failed to appreciate the implication you are suggesting for Obama.
The disclaimer that Ayers and Obama placed over the whole book means that Obama can deny anything in it that might make him look bad. Dreams is NOT a legal document or admission that could be considered evidence in any court.
Ayers, ever pushing race-victim status for blacks, might have made up the whole thing about how the children of the white mother got the estate and the children of the black mother didn't. Does that even make sense in post-colonial Kenya? Obama's relatives were paid off to keep quiet and telling the truth about discrepancies between Dreams and the truth is obviously the reason, IMO.
Dreams is so Ayers-infested, I don't take any of it seriously as fact.