User blog comment:Hawki/Nova Covert Ops, part 3 - Reflections/@comment-72.197.211.140-20161201222040

I'm not here to deny a viable ground resistance, Hawki. But the fact still stands: NO ONE expected the Tal'darim to suddenly show up and start attacking the world. Given their progress in flooding into the city at the start of the mission, it seemed that the Dominion defenses were quickly overrun.

On the other hand, it's entirely possible to believe that Davis did put defenses on Vardona in the event the Tal'darim DO show up. If that's true, then it means she thought ahead of the game and didn't underestimate them, maybe even having put up the defenses before drawing them into the conflict in the first place. If that's indeed what happened, then you can argue that those defenses kept the Death Fleet busy and therefore they were unable to purify the whole planet (assuming the Tal'darim always had the means to).