Both sides gain from trying to make the other side overextend. Nato "promised" Russia they would not send troops directly yet what do you think happens if Russia were to rush their whole army in and overextend? Similarly, the situation will improve even more for China should Nato get stuck in a kinetic war with Russia inside Ukraine.
Policies can be easily changed, especially when they're ambiguous to begin with. China's policy since the start of the conflict is to not sell it's own weapons to either party, but that can just be changed such as NATO's policy of not sending fighters and tanks. Also China's policy says nothing about volunteers or donated equipment.
Beijing would not give up the opportunity to bleed Washington in Ukraine.
