The fact that the film is 55 years old means most visitors here have not seen it (this isn't a classic films forum and the majority of HN readers were born long after the film came out), and the quality of the discussion here increases the odds people might be interested in seeing it. Why spoiler the film for them by needlessly including a detail that's a plot surprise but irrelevant to the subject matter of the discussion here?
The commenter must have believed -- and I tend to agree with them -- that it improved the quality of their comment. I'm sympathetic to the notion that one shouldn't needlessly spoil twists of movies, books etc when they're relatively fresh but at a certain point if we can't discuss what is interesting and fresh about a piece of culture we'll fail to make new touchstones for the culture.
