Pagan=people who worship multiple gods. Heathen=someone who's beliefs/culture you falsely believe to be inferior to your own. Both these arrogant assholes should be thinking of each other as heathens, not pagans. Wasn't sure at first if it was translator error or the authors cultural misstep, but from the state of this thread I'm guessing it's the author.
*shrugs*
Cultural missteps are common when reading foreign literature that deals specifically in cultures similar to your own. It tells you a lot about the people writing it, not about the cultures themselves and that's a valuable (and sometimes entertaining) insight. That's a good portion of the fun in these things tbh.