Since Remi hadn't sensed any magic being used by anyone during Felix's match with Julien, then that must have been so. Felix trusted the fox, at least in that aspect. Remi had no reason to lie to him about it, and as for his abilities to sense magic… He had even sensed the lingering magic traces after his mother had secretly stopped by to talk to Felix back in Silverfall, even though Agnes tried to conceal it, so his sensitivity must have been great. Felix had no magic abilities on his own and still didn't know all that much about the spells, so he couldn't make a good, informed assessment, but he had confidence in Remi (when it came to his abilities as a witch, at least).