What is the Chinese ADIZ?
An Air Defence Identification Zone is like a WARNING ZONE, not a no-fly-zone or an extension of Chinese airspace. An ADIZ is still international airspace.
Around 20 countries have their own ADIZ but not every ADIZ has the same rules.
An ADIZ acts as a THREAT ASSESSMENT to the Chinese mainland.
If an aircraft enters the ADIZ and identifies itself, then China knows what it's doing and that it poses no direct threat to the Chinese mainland.
If an aircraft enters the ADIZ and don't identify itself, then there are few scenarios that determines whether China should deploy its fighters to intercept the aircraft.
If an aircraft don't identify itself but merely skirt around the outer edge of the ADIZ like what the B-52 bombers did, China has no reason to deploy its fighters as those B-52 bombers poses no direct threat to the Chinese mainland.
But, if the B-52 bombers don't identify itself AND keeps COMING TOWARDS the Chinese mainland, then China will do a threat assessment of the aircraft and deploy its fighters to intercept the B-52 bombers.
Understand that China will only deploy its fighters based on the threat an aircraft poses to the Chinese mainland. If the aircraft identifies itself, China knows what the aircraft is doing and that it poses no direct threat. It is only when an aircraft don't identify itself AND keeps coming towards the Chinese mainland, then China has to assess the level of threat the aircraft poses to the Chinese mainland, if the threat is determined to be high, then China will deploy its fighters to intercept the unidentified aircraft as it poses a direct threat to the Chinese mainland.
This is essentially what the Chinese ADIZ is all about.