I am not positive but pretty sure that those seats weren't actaully sold out, SWA is just not releasing all flights/dates on their ding fares. Especially popular flying dates/flights since they know they will probably fill the plane without a ding fare.
I have checked ding just seconds after it was released and saw a lot of "not available" on popular dates/flights.
Any body else wish to comment on this? this is just my opinion, I could be wrong.
Suzy V.