In theory, but calling isn't always an option with SWA to get the lowest rates. Live agents cost money, so many companies prefer to have customers book electronically. Just as WDW is moving in this direction, SWA has long been there.
Now to be fair, I've never actually booked a vacation via SWA vacations.
Because the general caveat when it comes to packages like this is that they are non-refundable. Most often, you get the same prices but much better terms booking your air and hotel via each website. Actually, most companies tend to dislike third party websites altogether.
In general, companies make more money when you use their sites, so they reward loyalty with better customer service.
That's not to say one should never take a deal through a third party, it just pays to know all the terms.