Unclear to me how this fanfic concept could apply to a character in an official movie.

How could this be unclear? They are both narratives. They both have characters. Why wouldn't a concept about characters from one map to the other? Do you think fanfic characters and official movie characters are incompatible types? I have never read or written a fan fic and encountered this term countless times in discussion of media.

I'm honestly baffled how you can't imagine a movie with a character embodying the qualities described in the tvtropes entry.

I'm going by the description in the source you provided me, which provides a detailed and specific meaning in the context of fanfic and then says the term is also used as a general pejorative. If I'm not meant to use the description in that source I'm not sure why you linked to it.