"Magic?" Magic wasn't real where he came from. Well, there were so called magicians, but they used sleight of hand and other distraction and illusion techniques. Real magic, like what Klara described, wasn't real.
But Madison Valley was a strange place, and Evan wasn't so much questioning her explanation as seeking to make sure he was understanding correctly. If she said she could use magic to make the plants move and the fire keep itself contained, he'd believe her. He had no reason not to.