It's not a "demo" if you think that. And It's not the full version either. Because, the Open Beta is still under development, and each 2 weeks new patches comes out for this mod. In any case, you can fully download the Open Beta completely for free without any costs, and when the "Full" version gets released (when it gets done, because it might take a while because it is still under development) the game will of course still be free.
Either way, from long story short. The Open Beta is "fully" downloadable at its current version.