This is because the one-drop rule had anyone with known "black" ancestry categorized as African-American. The mixed race people on the end of the African-American scale have skewed African-American genetics a bit. I shared a page of African-American genetics a while back, and there were many people that had African DNA in the high 80% to mid 90%, especially among people from South Carolina and Georgia as the article rightly says. There were a number of people with Native DNA as well, and they were about 1% or 2%, but majority had none at all.
I'd say the article is on point minus the European part, because mixed race people i.e white + black or any combination of AfrAm and another race tend to report themselves as being "African-American", skewing real African-American DNA studies. They found people with as high as 75% European DNA identifying as being African-American.