Yeah sure, you can cut the cables. But like you said, alternate means of accessing the information exist as well. Technology limitations can be very difficult to enforce and maintain due to how quickly it can grow and evolve if people are motivated enough. Which the North Korean government definitely would be.
That’s true for pretty much all countries. Virtually all countries have a few sites that are responsible for routing the majority of the countries Internet traffic. The trick is keeping it that way. It’s very easy to throw in some kind of band-aid solution to get it back up and running if you are able to throw enough resources at the problem.