Games don't have a minimum "bullets per minute" quota they need to fill to be considered a shooter. Sometimes I go on a melee spree in Halo, does that make it "not a shooter"?

SS2 lets you use projectile weapons as your preferred way of disposing of enemies. In my mind, if you choose that path, it could be described as a shooter. That isn't all it is, but it is one word that describes a particular way you can choose to play the game.

Side note: SS2 is probably my favourite game so I certainly don't mean to "degrade" it by implying it is "just" a shooter.