Again, your information and logic are wrong. I think China is still using 大陸架延伸法 in East Sea which includes Diaoyu islands vs Japan's 中間線.
If China uses 中間線, then there would not be a dispute over 春曉油田. The fact that there is a dispute shows that China is still using 大陸架延伸法.
Also, the fact that Japan agrees to 共同開發 春曉油田 means it is Japan that is contradicting itself and is showing signs of giving up on East Sea and Diaoyu islands.
The Japanese are scared now that mainland China and Taiwan are controlling the Taiwan Straits, putting a squeeze on Japan's oil supply from the Middle East.