I'm sure she was right in theory, but clearly had no idea of the practicalities of how immigration is set up at the airport.

In any case, I doubt very much Air Asia would check you in online without requiring you call by their check-in desk at DPS. They're responsible for checking your eligibility to enter Thailand.

Unless you are remarkably fortunate, you'll have to go through the arrival hassle, purchase a visa, and check-in landside, so I hope you have allowed enough time to do that.