China does have some say over North Korea. Tens of thousands of North Koreans work as slave workers in China, where the bulk of their earnings go the the NK regime. Cheap labor for China, big income for the NK regime. President Trump signed a law in August forbidding U.S. products from being made by North Korean workers, but the reality is that some goods coming to the U.S.A. from China are made by North Korean workers there. Cheap food, clothes and other Chinese products are cheap partly because of the imported North Korean workers.
That will drive home the message to Pyongyang elite that they can no longer count on China's help or acquiescence, forcing them to reevaluate their future.