China is also the largest trading partner of every SEA country IIRC. Plus China is the world's largest trading nation with most of this trade passing through the SCS. So China wants any and all trade to flow through the SCS with no interruptions.
In the event of a US-China conflict, it will be the US Navy trying to blockade China and cut off trade in the SCS. Note the littoral SEA nations really depend on this trade.
So further Chinese military capabilities means the Chinese military is better able to protect these trade flows from the US Navy.
Remember that countries representing about 1 billion people (out of 8 billion) have placed any sanctions on Russia because of Ukraine.
The other 7 billion people live in countries that haven't placed any sanctions.
And again, the Chinese Communist Party founding myth is against nasty colonial powers like Imperial Japan or the British Empire. Such an ideology is innately anti-expansionist in terms of warlike imperial conquests.
Not only that, at this point China's the last one able to break the Western imperialist stranglehold over the world economy and providing space for developing countries to grow instead of being permanent economic vassals to said imperialists, so China must be ready to fight especially given the devastation already wrought by the imperialists (e.g., Iraq, Libya, Syria, Afghanistan etc.) and of which the Russians and Iranians are already resisting.