The coal ban began in February, I just found this:
China’s move to ban coal imports from North Korea, effectively slicing the country’s exports by about half, came with a message for the U.S. and its allies: It’s time to do a deal.
Authorities in Beijing announced on Saturday that China would halt all shipments from Kim Jong Un’s regime until the end of the year, in compliance with United Nations Security Council resolutions over North Korea’s nuclear program.
At the same time, Chinese officials said that pushing North Korea into a corner won’t work as Kim’s regime will keep developing its nuclear capability until it feels safe. Instead, it’s time to restart talks and “break the negative cycle on the nuclear issue,” Chinese Foreign Minister Wang Yi said in a statement on Sunday after meeting South Korean counterpart Yun Byung-se at a security meeting in Munich.
“The Chinese are getting more frustrated with North Korea,” Eurasia Group President Ian Bremmer said in an interview at the same conference. “They clearly don’t feel that they have a lot of influence and they’re worried that the U.S. under Trump is going to blame China as opposed to continuing a multilateral process.”
So probably China is in on this from at least some months ago. The problem, is if North Korea is willing to make a deal.