The article says more, but he quotes this analyst- Airline analyst Bob McAdoo of Avondale Partners- :

"We believe the B717s will eventually fly for Delta. Southwest wants to keep a single fleet type and, as indicated in this article, and will be pulling flights from smaller cities. Delta just ordered 100 larger B737-900 aircraft of its indicated 200 aircraft requirement. "We believe the 86 B717s at AirTran, currently on lease from Boeing, will be traded in to Boeing for more Southwest B737's. Expect Boeing to then lease or sell the B717s to Delta, perhaps as a second, but yet unannounced, part of the recent B737 order from Delta to Boeing.
"This would be consistent with DAL's plan to limit capital expenditures while opportunistically upgrading the Delta fleet. It would also explain why DAL did not announce orders for smaller aircraft to satisfy part of its 200 aircraft total need.
"This decision by Southwest and our assumed moves by Delta would both seem be in the best interests of these two airlines."