It's because people forget that China has about the same population and industrial output as NATO, and around 65% of their gdp, which if they bothered with some creative accounting tricks could easily be 100%.
US is more scared of being isolated than China, because US doesn't directly compare to China. NATO does. And while NATO would still prefer to have some non NATO partners, it isn't reliant on it. The same applies to China.
People have this very very wrong perception that global south countries such as the likes of Iran serve a role similar to France or UK in China's alliance structure, when in reality, the role of France or UK in China's alliance structure is served by say Chongqing or Guangdong. Iran, the various sahel anti French dudes, are more like Pinochet or Milei. Nice to have, but the empire does not survive off their contributions, and new ones similar to them are quite easy to create.
Russia is the only actual essential ally to China, because it ensures resource autarky in the worst case scenario. That's why theyre heavily backing Russia.