Why would LOST with our participation be any more important to China than any other treaty they ignore?
China has refused to abide by any international agreements when it comes to their claims on nearly all the South China Sea. As far as China is concerned the area is owned by China and China will seek to establish control over it all as peacefully as possible.
For China's rulers, paper is useful only to confirm Chinese ambitions. Power and not paper will constrain Chinese ambitions.