Clockwork Prince question

I think it said that no one knew exactly what the fair folk were, but there were multiple theories including them being the direct offspring of demons and angels. Yeah, that does seem contradictory, though. Maybe, since that theory was told during TMI, they find out something in TID that makes that possible, just not well known. Or Shadowhunters and demons doesn't work (since the result is stillborn), but angels and demons does. So confusing!