"God-Damned Yankees" is a very specific subset of "Them Fellers." So if the Zeppelin Truppen crash through the window, steal the plans, and then escape on zip lines, Private Hiram Hickam from Possum Holler, Alabama will say: "Them fellers done runned off with the plans!"
If the plans were stolen by upper-class Cthulhuian cultists, it's entirely possible Private Hickam will say: "Them God-Damned Yankees done runned off with the plans!"
It is likely that Private Hickam has a higher opinion of the Zeppelin troops than he does of anyone from New England.
I lived in Tennessee. I have heard men say "God Damned Yankee" and not jokingly.