In answer to your inquiries:
After the discovery John's mother agreed to be tested by 23andMe, which confirmed that John's African ancestry comes exclusively from his maternal side. His mother's Ancestry Painting shows ten percent African, which means that John must have African ancestry from maternal lines other than the Hemings line. Since it is believed that the Hemings/Jefferson children were only 1/8 African, John's (and his mother's) African percentage would be far lower if that were the only line contributing to it. Since Madison Hemings chose to live as a black man in black society, unlike his siblings, it makes sense that his descendants' DNA shows evidence of contributions from additional African ancestors. This is evidenced by John's maternal DNA haplogroup L3e4, which would have been inherited not from Sally, but from Madison's wife Mary Hughes McCoy "a free woman of color" who, like Madison, was of mixed race. Mary had at least one white grandfather, the planter Samuel Hughes who had children with her grandmother Chana and freed her from slavery. John's colorful Ancestry Painting illustrates this tradition of slave owners having domestic relationships and bearing children with their slaves.
I am receiving a lot of responses to the original post and will continue to follow-up when appropriate.