His words speak volumes. Most Western leaders misinterpret his words to mean that China is thinking about cracking down on North Korea's nuclear and missile ambitions. In truth, North Korea has always been China's useful idiot, a buffer zone against the West.
He also promised that online criticism towards the Communist Party would be accepted as long as it was "well-intentioned" regardless of how unpleasant it was to hear. The state news agency Xinhua posted highlights of the speech on its Weibo account, with Weibo being China's blogging service. However, the only two published comments were from sycophants, with one being: "It is a good channel to listen to public opinion through the Internet. The most important thing is to safeguard the country's cybersecurity and a clear environment for public opinion. Those distorted, slanderous improper comments should be eliminated."
Mao Zedong, the first paramount leader, famously declared that China and North Korea were as close "as lips and teeth."
"Korea is China's neighbor... The Chinese people cannot but be concerned about a solution of the Korean question," Chinese Premier Zhou Enlai told the UN on 20 August 1950 as UN troops were advancing toward the Chinese border. Then in November 1950, Chinese troops attacked en masse, forcing the third change of possession of Seoul since June 25, 1950, when North Korean troops crossed the border separating the two Koreas, starting the war China still refers to as The War to Resist American Aggression and Assist Korea.
Xi was warning the U.S. to stay out of North Korea regardless of what happens.
The U.S. has naive plans to secure WMDs via the 8th Army, nuclear disablement teams, and special forces, but China would view these maneuvers as a repeat of the second half of 1950. As I wrote in Could an implosion of North Korea result in WWIII?, there is more at stake than WMDs, important though they may be. The 120,000 to 200,000 labor camp inmates are at risk of being slaughtered by guards worried about being tried for war crimes. Millions of starving refugees would stream into Russia, China, and the ROK, across land mines in the last case. Some in South Korea and China believe that setting up machine guns is the best approach for putting a halt to refugee movement, but that would poison the well of relations for generations.
There are only two ways of avoiding WWIII in the case of a DPRK implosion:
1) A group of North Korean military officers contacting the ROK and arranging a managed implosion, starting with the assassination of Kim Jong-un and as many toadies as possible.
2) Promising China that U.S. troops will not move north of the 38th parallel and will only serve in a support role, but in return crafting detailed plans for an implosion, especially with respect to humanitarian issues.
The first is the only way to avoid mass bloodshed, though the monsters running the labor camps would need to be eliminated as well as the leadership. However, the second would only work if China and Russia do not interfere with the ROK's actions to reunify the peninsula. The U.S. has the ultimate leverage against China, the largest trading relationship the world has ever seen, albeit mostly to the benefit of China and multinational corporations such as Apple, Home Depot, and Walmart. If we closed our borders to Chinese goods, China would be the country to implode. Never in history have two countries been tied together with golden handcuffs, with one side afraid to offend and the other side fortifying its defensive positions for the upcoming conflict.