What they mean is their code and assets. Anything made within inside of the game it is completely fine. That agreement is for their code assets. Meaning if someone were to take their code and remake minecraft then that is a problem, but if it is something made within the game and someone sells their map then its fine because its not a copy of their actual game code. Minecraft is a sandbox game so they want people to build create hell even make money off what they made. If Mojang had a problem then 60% of the community would have to stop making maps and selling them. Mojang doesnt have a problem with people doing this so its legal if it is made with inside of the game.