I do not think China will directly involved with US militarily.
If we posit that PLA feels the chances of US winning a shooting war inside SCS (which is entirely within the range of DF-26B and mostly in range of DF-21D) is at best even then this wave of greyzone warfare / People's War makes a lot of sense. It's not at all targeted at Philippines but instead a bait for the US. If the US does not get involved and they leave Philippines high and dry then globally it will be perceived as US showing their weakness and possible start of a retreat back to isolationism. Countries that depends on US for defensive pacts might rethink their strategy and some might even swing around to China' side.
if the US takes the bait and does get involved militarily than they will be fighting in China's home turf and facing all sorts of carefully laid out traps for them there. If the US loses than they again will be forced to retreat from South East Asia.
So US hegemony is threatened if they don't do anything and is also threatened if they do something but loses. They have to win a stand up fight in SCS to come out ahead and that's far from certain.
China may use the north Korean strategy.
If US attacks NK, NK will fire at SK.
If US fires on China, China will fire at Taiwan.
China and Taiwan technically still at civil war