During matches, I will occasionally pause to do something else (work, chat, etc.) and then pick the game back up and continue. Given that some matches take 8-10 minutes to complete at the highest difficulties (like the Level 85 dungeon challenge today); this can result in several interruptions to my match as I pause to do something else (likely to return in a few minutes). As a result I have been disconnected twice now due to inactivity while literally playing the game with brief stops inbetween.
Clearly, whatever detection system for activity exists does not actually include the action taken in the match itself otherwise I would not be disconnected for inactivity.