Except China has been working on anti-carrier weapons and tactics for at least 20 years now. If shit stayed conventional, and it looked like NK was gonna get steamrolled, I could see the PLA (with or without official sanction) handing the DPRK a few under the table to help even the odds (and blunt US force projection at the same time).
Of course China's been working on anti-carrier strategies and weapons. That would be a huge deal,
in a war with China.
At present, the known Chinese approaches to anti-carrier work involve anti-ship ballistic missiles and sea-skimming cruise missiles, the same as everyone else who plots attacks against carriers are using.
Sea-skimming cruise missiles would have to be launched in very high numbers (at least 60) to have a real chance of breaking through a carrier battle group's defenses, as the USN (and allied navies) has spend the last fifty years perfecting defenses against precisely that threat. Surreptitiously supplying the DPRK with that many weapons would be very difficult to pull off without getting caught. This is also a threat that can be completely neutralized by staying far enough offshore, as the DPRK doesn't have sufficient mobile launch platforms (bombers, missile boats, etc) to carry out such a strike.
We don't know how well a carrier group's defenses would stand up to a ASBM attack, so it is conceivable (but unlikely) that a mere dozen or so missiles might get through. This is, however, largely irrelevant. Only four nations are working on such systems (Russia, China, India, and Iran), and only one has one that would be useful in this scenario (Russia cancelled their system under the SALT treaty, India's and Iran's are pretty short ranged). It is literally impossible for China not to be immediately blamed if such a weapon destroyed a US ship.
Openly providing the DPRK with such a weapon would be a breach of a great many international treaties and cause immense diplomatic damage to China.
Providing such a weapon to the DPRK in secret, during active or imminent hostilities, for the express purpose of causing damage to a rival would fall under a much simpler category. It would be an act of war.
China's government is not nearly stupid enough to take that risk, and if some lower-level official decided to do so, that official's family would almost certainly be billed the cost of a bullet.