Yes that is true. Atts does have its own ecu; but there is only three reasons why that light should come on:
1. when you first start the car (then it goes off)
2. something is actually wrong with the unit and its componets (which in this case the dealer didn't find any)
3. if there is a check engine light atts is disabled until check engine problem is fix.

Thats why I asked if he has a check engine light becuase if he is not throwing a code for the atts unit, the only other option is a cel code.