China is an anathema for liberals because it's a tangible real world demonstration that viable alternatives to liberal ideology are possible. The whole argument for liberalism is premised on the idea that liberalism sucks, but everything else is worse.

Liberals promoted western model as the only viable system going forward ever since USSR dissolved. Any country that deviated from this model was painted as being backwards and a type of country you wouldn't want to live in. Now we're seeing China developing rapidly and going from strength to strength. China doesn't suffer from the economic crashes the west has once every decade, it's continuously improving the standard of living for its people, it coexists peacefully with other countries. It's an example of an alternative model to liberalism that demonstrably produces better outcomes by pretty much every metric. This is why China is such a threat to liberals, it blows apart the argument that nothing better is possible.