I've seen two reviews that show that this store page is not as clear as it should be.
Let me explain: This is a working multiplayer FPS. By working I mean, you can host your own server, and have people join it (by sharing your IP address). There you can kill (2) zombies or other players, pickup weapons, health and walk around the map.
But this isn't a full game. This is just the build of what YOU (yes you) can create by watching the video series "How to Create a Multiplayer FPS" linked in the description.
Finally, let me put it in this way: This only shows the gameplay you can create on your own (by watching the videos that explain everything), and with some other models, menus, sounds, and those "details", you can have your own Multiplayer FPS up and running :)
Hope you enjoy this demo, and if you decide to check the series, hope you enjoy them as well.
Any questions just ask on the discussion forums :)
Kind regards,
Vitor.