In addition to that, I believe all SW passengers need to check in on the domestic side in ATL. See this thread.
That's correct, Southwest does not have any ticket counter or baggage capabilities in the ATL international terminal.
I guess to clarify this is if you are checking in - for a passenger that has mobile boarding passes and only a carry-on luggage, you could come in the International terminal and walk/plane train back to the C concourses.
Maybe some hotels have shuttles that stop there first or some other reason that it would make sense. But anyone checking luggage or that can't download a boarding pass in advance should only use the domestic terminal for Southwest.
Home airport MDW, frequent visitor to MCO to see the mouse.