Despite our differences in social upbringing, nationality, language, religion, etc... We all agree in 1 thing, that Die Hard is a Christmas Movie, right?
Yippee Kay Yay... (The only movie that doesn't depresses me in Xmas)
I don't wanna be the party pooper here, but Die Hard is a specifically American Christmas Movie. I'm not saying that other people can't or shouldn't enjoy it as a Christmas Movie if so inclined.
But as a German what we "traditionally" watch as Christmas Movies/Shows is something completely different (though also varying greatly from region to region and family to family.
I for one still enjoy "Weihnachten bei den Hoppenstedts" a lot my father mostly insists on seeing "Familie Heinz Becker feiert Weihnachten".
A movie a lot of people consider to be a Christmas Movie as far as I can tell is "Drei Haselnüsse für Aschenbrödel".
Again I'm not saying that people can't consider "American" Christmas Movies as their own or make them their traditions a friend of mine insists that "In Bruges" is the ultimate Christmas Movie. I just think saying "We all agree" is making it to simple
Edit: Read some more comments and maybe I misunderstood the premise. I'm not saying "Die Hard is not a Christmas Movie" what I'm trying to say is "Christmas Movies are what people make their Christmas Movie regardless of if they are Christmas-y" if I started watching Shrek at every Christmas it would be my Christmas Movie. But I feel what OP wanted to say was "There are Movies that are considered Christmas Movies because of the content and Die Hard should be considered one of them".
I think the joke is that Die Hard obviously isn't a traditional Christmas movie. But the story is literally set at Christmas. Many people like Die Hard more than traditional Christmas movies, so when their family members start voting om which Christmas movie to watch this year, they steadfastly nominate Die Hard with the argument that "it really is a Christmas movie, the internet agrees"