Thats just bullshit, zombies is just running and running in circles, buying new items and shooting 30 bullets to one piece of flesh. It's fun, but it is absolutely not difficult.
Multiplayer requires skills. Let me give a proof: You might want to run around in MP too, but you won't get 2500+ kills like you do in Zombies. Without tactical skills, you wont get any points in MP. Just slashing for kills is ineffective, play the objective. If you play alone, don't go to tactical gamemodes or you might not do well. Use communication and teamwork to play the objective. An absolute truth is that even noobs can dominate if they communicate.
Everything you do in MP requires tactics and skills, playing the objective and staying alive requires tactics. You can play it without using any tactics, but then you most likely fail. Zombies has nothing to do with tactics, its just a run n gun session.