For years, China told the Bush and Obama administrations that it had limited control over North Korea. That seemed a bit hard to believe, but was not widely disputed.
Notice this sequence of recent events:
1. Trump links North Korea and trade in discussions with Xi.
2. China takes some steps to rein in North Korea but only limited ones.
3. Trump announces new tariffs against China.
4. Kim and Xi meet. Kim announces he wants to end war with south and denuclearize Korea.
5. Trump does not pull the China tariffs.
6. Kim and Xi meet again and immediately afterward North Korea returns to hostile stance, which leads to Trump pulling the planned summit.
It would seem obvious that everything Kim has done has been at the direction of China. This sequence of events seems to prove China’s prior claim of not being able to direct North Korea completely false. It also seems clear that China is playing North Korea as a card to achieve other ends, especially in its trade negotiations.