Responses
I actually kind of like the idea of him not knowing everything, because it really puts it into perspective of it actually being its own world with a much bigger story then what was already written by him. Anything he didn't write in detail was essentially just gaps that were filled in.
With the comments saying that the mc as an author should know their characters inside out just hits me because even I forget that I'm a fanfiction writer and then when I randomly remember I have an unfinished fic that I haven't updated for many months I have to reread it again to know the details since I could only remember the plot not the details. I also keep some notes since I forget some characters I made even exist and embarrassingly have my own reader call out like 'What happened to person A? To person B?'
So I think we should cut the author mc some slack. He was literally forced to write that novel as he said in chapter one which means he had no passion in making any of the plot or characters. I bet he just made OGAdel's backstory vague because he's too lazy to write him for more depth and just made him a 'vengeful villain who loves to kill'. And now he was thrown into the novel where all his characters are brought to life it's natural even he doesn't know anything about the details of the demons since he himself only written them off as 'demons' and nothing more and focus solely on the humans and protagonists. If he did said he was passionate about teh novel then it's valid to think making him author is just an excuse for his cheat items.