It's not a movie about Christmas, but it is a movie set at Christmas that has a tradition of being played at Christmas.
It is a Christmas movie, simply for the fact it has become one. You don't get to be arbiter of what is and isn't, the people have spoken.
Depends on what people mean when they say 'about Christmas'. Like Lethal Weapon is about Christmas in the sense that it's about someone feeling suicidal around the holidays because they lost a loved one.
Like I joke mother! is more of a Christmas movie than Elf or A Christmas Carol even. Cause it's a birth of christ movie.
But thematically anything about family and community, charity and togetherness set around christmas is about Christmas in the secular sense (and that's at the root of A Christmas Carol, not the tiny smattering of references to how Tiny Tim loves Jesus or whatever).
Die Hard 2 is about traveling to be with family, which is inherently Christmassy if you ask me.
Shazam is extremely Christamassy, thematically.
But I also think films about loneliness and over commercialization etc are about Christmas if they are set at that time of year.
And speaking personally if your film centers on a Christmas tradition like gift giving or Christmas parties, like Die Hard or Go, or a host of other stuff, it should count.