You're implying ASEAN members all act in unison -- they do not. They fight amongst each-other almost as much as they do with China.
And while they fight amongst each-other, China voices support for one while undermining that support in another fashion. Pitting them against each-other is common.
And while they fight amongst each-other, China voices support for one while undermining that support in another fashion. Pitting them against each-other is common.