I could see why the author might have gone with a word that means king, given that they're Japanese. "Maou" is still basically "magic/demon king" even when they're applying it to female characters. Several nicknames I've seen in series also were calling female characters a "king" of something or other. I'd say novel and manga makers from Japan are frankly a little bad at foreign languages, and rule of cool is the order of the day anyway.