I've not seen anyone say that you should get this for the singleplayer alone, only that it's an OK campaign in a multiplayer focused game.
I actually think it's good that the campaign is short since it is, again, a multiplayer focused game. And having a campaign built as a tutorial with some story is perfectly fitting for a game like this.
Pre-alpha refers to all activities performed during the software project before formal testing. These activities can include requirements analysis, software design, software development, and unit testing. In typical open source development, there are several types of pre-alpha versions. Milestone versions include specific sets of functions and are released as soon as the functionality is complete.