A flat character is a character that does not change throughout the book or movie so even though I have not read that book or watched the movie, I would guess it is because he does the change throughout the book. Hope this helps! シ
King Hrothgar in the epic story Beowulf, is considered a flat character because when the story is being told, he is an old King. With that being said, Hrothgar has already lived and reached maturity so he doesn't change or develop and he is not complicated.
