When in his human form, Remus was able to understand magic quite well. In fact, he could boast that he was one of the brighter students at Hogwarts, even though he really wouldn't. He had done most of the work when putting the Marauder's Map together and he had also helped James, Sirius, and Peter figure out how to become animagi. That completely changed when he transformed into a werewolf.

Thus, the fire coming towards Remus took him completely by surprise and he was not ready to react to it. Before he could react, the fire reached him, scorching his fur and burning his face. He immediately turned away from Siax, running away from the fire. Whatever this guy was, he was not worth the risk of taking down, and Remus fled from the fight.