Probably one of the simplest ways I've seen to explain a super-complicated concept. Very easy to understand, impressive. Also it's now confirmed that Ontan did in fact doom the world. Might help explain how she was so calm about learning about the world ending compared to Kadode. Not sure she consciously remembered all this, but her subconscious already knew there was a price to this.
I'm thinking the "Dead Dead Demon" in the title refers at least in part to Kadode's old nickname from her bullies and that she died. The double-dead part of the title makes me wary that she's going to have to die again in this timeline, too.