First of all, having the genre name in the title doesn't mean it can be categorized as such. Isekai Quartet isn't Isekai.
Kenja no Mago is just...no, reincarnating into another world doesn't make it Isekai either. It's a dumb 1 minute framing device that has nothing to do with the rest of the story. That's what the genre is categorizing: the story. Not the setting. It's almost similar to Knights & Magic. But in that show the MC's memories, computers skills, and mecha obsession are what drive the story. But judging by the first episode, it's going to be another Irregular at Magic School.
