Thinking about it a bit, this is the most likely way the author could bring back the supernatural premise of the story.
It took me so long to think of it because the supernatural elements haven't come up, beyond the obvious, since the first chapter. But magic is real in this setting and if that part of the legend is true, why not the demons too?
Then there's Ranna's.... well, lack of personality. She's portrayed as a superhuman destructive force whose only desire is to fight, beyond Rokudou's effect on her. If demons exist they'd probably be like her, so why not have her actually be a demon?