China has been less aggressive with Indonesia than most of the Nations around the South China Sea.
Indonesia is relatively big in population - over 267 million people, it is the world’s 4th most populous country as well as the most populous Muslim-majority country.
It has the 14th biggest land area, and sits like a fence blocking Chinese access to the Indian Ocean, and Middle Eastern oil and gas. Also, it has the 16th largest GDP.
Indonesia has a history of violence against their Chinese minority population, and communists, who tried to take over the country by force (and were supported by China). When I say violence, I mean killing over a half million people, and purging communists so hard, that it leaves Beijing stepping carefully a half century later.
Indonesia is ready to flare up against China and ethnic Chinese, at the drop of a hat.
They have some jihadi preachers who would love to cook up 9-11 style attack on China over their oppression of the muslim Uighurs.
With Indonesian participation, the USA could implement an effective oil blockade on China, covering all the narrow straits with land based anti-ship missiles.
China needs to step carefully around volatile Indonesia sensitivities - it is taking longer to buy off their elites, except for the ethnic Chinese.
There are two basic ways to get people to cooperate with you. One is to develop a relationship that is mutually beneficial so that helping you helps them. Another way is to threaten them. China has threatened virtually all of their neighbors, starting with the attack on India which was probably intended to generate an outside threat to take the Chinese people’s attention off the failures of the CCP. That failed. China has also constructed dams along all the rivers that run through China and then flow into their neighboring countries. This gives them tremendous influence as they can starve those countries for water. The Chinese have threatened Japan and Taiwan. While Taiwan is no big threat, Japan has started an arms race with a huge increase in military spending. Then there are the almost constant threats to America.
I think that if a conflict breaks out we will see the neighboring states form a cohesive block. The neighbors will back Tibet, the Urgurs and Hong Kong separatists. China, who has actively been looking for an external enemy, will find a plethora of them.