Maybe because his narrative only focus solely on the protagonists and the human world and did not make any narrative or details about the demons or anything related to them except them being a villain.
Authors only typically write a character to an extent. Seldom do you see people write characters inside and out. Plus this generally all takes place in events and places with characters and such he didn't actively write about. He knows he made them but outside of that he doesn't really know since this is a "real" world now and not just his book. He doesn't have his reference sheets to make remembering easier either. So cut him some slack. Things are very different from just writing the baseline.
how did the bloody mc write the entire story and still be confused or unsure about some parts like??? surely if u wrote about a character u would know about them inside out? idk man