What the developers had proposed were two different types of worlds as @Jeffrotheswell stated. One is a rented planet that has to follow the rules of the live universe and is connected to the live universe. The second is a private planet that is not connected to the public universe because players can change the rules on the planet. So you are correct in that the rented planet players will be able to interact if they choose to but the private would not.
If I am understanding his point, by allowing players to buy private planets, you can let them change the rules they do not like. I think the hope is that by giving those players someplace else to play they will not complain as much in the forums. I am not sure that would work entirely given the fact that so many features like Titans are not even in the live game so players that have a private planet would have to constantly be adapting to new mechanics. This might be easier than I think as I have never tried to mod Boundless. And I think they will still want to comment on new features and since they paid for the game and for the planet they are playing on they should still be able to. There are also those that cannot afford to go to a private planet so this only solves the problem for those that can afford it or have friends that can.
Quite frankly if most of the population migrates to private planets and all change the same rule, then maybe that would provide feedback to the developers on what players really want and what might need to change to grow the game. If only a few migrate and the public universe starts growing then that is also good feedback.