It was implied the girl with the Doppelganger, Kuriba I think was her name, created Indian Poker back in the Railgun arc, but she was supremely sketchy about it and her explanation was full of holes. I was hoping they were gonna explain it in this manga, but it seems they're content in just referencing the main novels while going "wink wink nudge nudge" instead of actually building a cohesive story.