The international flights still depart from the domestic terminal.
The arrival flight since it is coming from FLL will also use the domestic terminal. (i.e. there will potentially be a bunch of people getting on in FLL that are not subject to customs - therefore you must be going through customs in FLL.)
I don't believe Southwest has any gates at the international terminal in ATL, so they must be either inbound connections through FLL, HOU, etc. or from pre-cleared origin airports.
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