Wasn't that the main cult in the castle, though? I am pretty sure another NPC talked about the castle in connection with the main cult (with which the other dragon riders in the game were associated).
It's possible that Rak'Sheen (or whatever the dragon rider's name was) was working for the "main cult", but I got the impression from various characters that he was something of a lone wolf who had gathered a cult of fanatical dragon worshippers for himself.
