there are already several threads on this topic.
anyway....
if this was the first Smash game, everybody would consider it a platformer, then discover it had awesome multiplayer in which you fought. (example 1: is vietiful joe red hot rumble wasnt its own game, just the multiplayer of viewtiful joe, viewtiful joe would still be called a platformer. example 2: halo is a shooter, not a party game. if h3 was the first halo game with a story, it would have been called a party game)
Action isnt a genre, it's basically being called that because if it said Platformer/Fighter it would seem like a random combination.