It's not. In 1.0, it will be.
Edit: you might downvote you all you want, it still doesn't change the fact that the game doesn't have a true offline mode. Don't believe me? Turn off the internet, launch the game and try to play.
Jesus Christ, this sub is frustrating. I can only imagine how devs must feel having to answer same questions every single day, but still it doesn't feel great being right when people don't even understand what "true offline" means