Those countries are controlling access at the very few origin points. And they can still be foiled by tunnels, VPNs, and Encryption. The only counter is to actually cut the network at that origin point. But that still gives a country sized internet that’s very resilient. Could they start isolating cities? Depends on their infrastructure. I know the mid size town I lived in could be shut down with one cable. (Because road construction hit it at least once a year and 80,000 people lost Internet for a couple days each time)

When it was first envisioned it was supposed to be an actual web. With multiple points of contact at each place. Instead we’ve consistently done the bare minimum to bring the Internet to each place. Meaning in many places there’s only one connection. For an international look at connection points there are undersea cable maps. It becomes clear quite quickly how easy it is to isolate a single country’s web.