I feel like there's more to the story here. If there were no more flights to OKC (and, looking at the flight schedules, that could be possible if they took the very last flight out of FLL), WN would have generally rebooked the passenger on a flight the next day pending room or offered a refund/credit.
From the Southwest Carriage Contract: (http://www.southwest.com/assets/pdfs/corporate-commitments/contract-of-carriage.pdf):
(4) Delays or Involuntary Cancellations. If a Passenger’s scheduled transportation is
cancelled, terminated, or delayed before the Passenger has reached his final
destination as a result of a flight cancellation, Carrier-caused missed connection,
flight delay, or omission of a scheduled stop, Carrier will either transport the
Passenger at no additional charge on another of Carrier’s flights, refund the fare for
the unused transportation, or provide a credit for such amount toward the purchase of
future travel.