Well, 1864 was long before the automobile. However, Atlanta, like most American cities that were getting big around 1900, had lots of streetcars, so it was never going to be walkable in the same way that European cities are. Despite the nostalgia for them, streetcars were definitely not as good as busses and cars - they were pretty slow and got stuck in traffic because they ran down the middle of the street without any barriers. The thing with cars is that they let people spread out much further than even streetcars could feasibly go, and as you said, Atlanta is pretty much the best-case scenario with cars.
Even LA became an automobile city, and it has much more substantial barriers to expansion. Now that it's built out, LA is developing a pretty expansive metro system, but it will take decades to truly cover the region. It would have been nice to at least lay the main lines of the metro when land was cheap. One thing China got right is learning from these mistakes and building out their metros ahead of their city growth.
Minor note that 2/3 of Atlanta was destroyed in 1864.
So in contrast to other major American cities, it had the benefit of being mostly planned in an automotive era.
Add in the fact that the surrounding geography doesn't preclude expansion in any direction, and it's essentially a "best-case" city for cars.