China is famous for their long term strategies. Allowing North Korea to intervene and Russia to continue its war efforts in Ukraine for China means weakening both of these countries. Direct Chinese intervention at this point is not necessary. Moreover, it is extremely effective as it allows to camouflage your policy behind peaceful initiatives (Chinese peace plan for Ukraine). The winner in this war will not be Ukraine/Russia (both are crippled and destroyed by the war), but the one who will bring Russia and Ukraine to the negotiating table and receive all the glory as a “peacemaker”. (We can just look at Trump).