Two years is long enough time to make a decent length game and multiplayer-- they just didn't spend their time wisely. I'm guessing Just about every texture and model in that game (except the hat box of course) is one shot. Instead of making a bunch of one shot models and textures, they could have done the smart thing that games like Gears of War and God of War did and was make a lot of reusable textures and models.
By doing that they would save A LOT of time doing textures and models and be able to provide A LOT more content for almost free (a level is probably in the area of 200MB since it's just a large text file).