It's not about disbelieving "every crazy story about every artifact." It's him denying the evidence of his senses and categorically stating that magic does not exist, which is blatantly, demonstrably untrue, even within the framework of this single story. He just saw Clark get completely blitzed by the mate of the amulet in question, and Clark recover from that change when exposed to the amulet that gave him the head-trip, and he's still standing here saying "the amulet didn't do anything to me, it can't have, because there's no such thing as magic."