The Chinese did not intend to watch as NATO forces approach their borders. They had a small portion of the Afghan border with them to throw their troops there.
After two days they were about 100km from Kabul ...

The Russians were in the dot. By helping the Chinese, they would face the US - and they did not want to. By acting against China's People's Army, they would become allies of the West - and they did not like it even more. Well, unless there are some loans ... or gas contracts ... So far, they just stopped sending their contingents to Tajikistan and Uzbekistan and watched how the competition was taking over. Exactly like Stalin in the 1940s ...
