This is correct. I emailed Air Asia and received the following response regarding the departure tax:

"Please be informed that the airport departure tax must be collected, which is paid at the airport usually upon leaving the country. The departure tax varies based on the specific airport and country.

Unfortunately Thai AirAsia do not provide a transit lounge so we cannot supply the service to hold transit passengers. We are a point to point service, meaning that our passengers have to verify at immigration point and check in at the counter again for a connecting flight.

Effective from 1 February 2007, the International Departure Tax will be increased from THB 500 to THB 700, and the Domestic Flights fee will change from THB 50 to THB 100. All charges will be included in the fare ticket."