This is only an argument because of different ways of defining what a "Christmas movie" is, which aren't mutually exclusive: a) Whether the movie is unambiguously set during Christmas, and if so whether that's clearly an artistic choice, or as much of a coincidence as a movie that has nothing to do with the weather being set in the summer or winter. b) Whether the movie is aimed at the "Christmas movie market". I.e.…
Also C: Its a christmas movie if we all decide it is. If it becomes one more of the christmas-centric traditions, then it becomes a christmas movie.
If "Die Hard" becomes a Christmas movie over time, it's a thing that happens. People are messy. Societies of people, exponentially so.