We can ignore current settlement patterns because Woodard does. In a recent paper (https://www.sciencedirect.com/science/article/abs/pii/S00330...) he does explain the methodology, although I don't have access - but from the snippets I can see it appears that he's essentially trying to work out who the first European settlers in each area were. So it doesn't matter that north Fulton County is full of carpetbaggers from up North and immigrants. (I write this as I sit in an office in north Fulton County; I am a carpetbagger from up North and many of my co-workers are immigrants.)
It makes sense for the split to be along county lines just because a lot of data will be available at the county level, but it occasionally produces absurd results. I occasionally have mocked these splits as "I drive to Appalachia for ramen", because I used to live in DeKalb County about a mile from the DeKalb-Gwinnett county line - according to Woodard's map, DeKalb is "Deep South" and Gwinnett is "Appalachia" - and I liked a ramen place just over the county line. (Since then both I and the ramen place have moved.)
