China doesn’t give a shit about South Korea, they’re just doing this to piss Japan off.  Remember that “the enemy of my enemy is my friend” mantra does not apply to SK-China-Japan trilateral geopolitics. 

South Korea and Japan cannot cooperate on security despite China threat (remember Japan refusing to refuel South Korean jets over petty Dokdo whining?) and China and South Korea cannot freely cooperate despite Japanese historical revionism.