Is The Nightmare Before Christmas a Halloween movie, or a Christmas movie? In terms of worldbuilding, it’s obviously both—it’s about a bunch of Halloween-town residents taking over Christmas from Santa Claus.
But worldbuilding elements don’t suffice as genre classifiers, or else black comedies wouldn’t exist. Creators deliberately apply worldbuilding elements from one genre to another for pure frission’s sake. Consider Kiss Kiss Bang Bang (speaking of Christmas movies), which takes a New York noir character, a down-on-his-luck con, and drops him into an LA noir scenario of movie glitz and private eyes; or Rian Johnson’s amazing Brick, a noir story engine driving high school characters. Fantasy literature is rife with this sort of behavior—consider Steven Brust’s use of crime drama story in the Vlad Taltos books, or for that matter the tug of war between detective fiction and fantasy that propels considerable swaths of urban fantasy. If we classify stories solely by the worldbuilding elements they contain, we’re engaging in the same fallacy as the Certain Kind of Book Review that blithely dismisses all science fiction as “those books with rockets.”