It would be a very neat trick to link the y-chromosome DNA to a progenitor without first linkng it to a first-degree relative parent.
If you are doubtful, I suggest that you contact the peer reviewer and ask her directly if the DNA analysis confirmed Stanley Ann Dunham as Barack Obama’s mother.
Here’s the contact and she has a web site:
“An expert in Southern research and past president of the Board for Certification of Genealogists, Elizabeth Shown Mills, performed a third-party review of the research and documentation to verify the findings.
‘In reviewing Ancestry.coms conclusions, I weighed not only the actual findings but also Virginias laws and social attitudes when John Punch was living,’ said Mills. ‘A careful consideration of the evidence convinces me that the Y-DNA evidence of African origin is indisputable, and the surviving paper trail points solely to John Punch as the logical candidate. Genealogical research on individuals who lived hundreds of years ago can never definitively prove that one man fathered another, but this research meets the highest standards and can be offered with confidence.’—from the ancestry.com press release on their findings
Bunche was born in Detroit, Michigan in 1903 or 1904 and baptized at the city's Second Baptist Church. His father, Fred, was a barber, while his mother, Olive Agnes (née Johnson), was an amateur musician, from a "large and talented family...
Fred Bunche is believed to have had Bunch and other ancestors who were established as free people of color in Virginia before the American Revolution. The Bunch/Bunche surname was extremely rare. In 2012 researchers published evidence showing that Bunch male descendants can be traced through historical records and y-DNA analysis to John Punch, an African indentured servant sentenced to life service in 1640, and considered to be the first slave in Virginia.[7] President Barack Obama is also believed to be among Bunch's many descendants.[7] Several generations of the Bunch men, free people of color, married white women from the British Isles.[8]