Coke started in Georgia, is huge across the south. Has marketed itself as like a family friendly or wholesome nostalgia brand for a long time. Which plays really well with the old white people.
Yes and those people who go to KFC are less likely to be Republicans which is what this graphic is breaking down.
Brands do demographic surveys all the time. Chick-fil-A is the top fried chicken place for self-identified Republicans. And also happens to serve coke products exclusively.