I know I am splitting hairs here. I don't think the Necromorphs are true zombies. They seem to be the worker-bees for some higher inelligences.
Much like the Aliens from, well... Aliens. The Bugs from Starship Troopers. Unless the Necromorphs attack anything, and everything that moves. Then they would be Zombies. However, if they don't attack things like the bigger monsters.