It's more than just people blindly killing zombies, it does require a lot of team work and planning to survive. You have to throw molitovs at door entrances to block zombies from entering, make sure you have someone with a shotgun covering your back as you move, then you hear a fucking witch in the room and are forced to turn off your lights and pray it doesn't hear you. The game is awesome.
It doesn't feel like a mod to me, and I'm not trying to say I'm the authority on this, but I've played countless numbers of zombie mods for PC games over the ears and this one is no mod. It feels so well crafted even down to the things your characters say.