One more pt @resplinodell's "realist" take on intl law, which I am generally sympathetic to. Even powerful states like the US find it in their interest to agree to legally enforceable maritime boundaries w/ smaller states. See https://www.state.gov/u-s-maritime-boundaries-agreements-and-treaties/

The US submitted to arbitration w/ Canada, has made maritime boundary treaties w/ "smaller" countries like Cuba, Kiribati, Micronesia, Mexico, Venezuela, etc. Just b/c China is powerful, doesn't mean it is other states' fault I won't enter into those deals.

Needless to say, the US is not always the "good guy" in intl relations or intl law. But I think many commentators in China and the US oversimplify the US relationship w/ intl law, even as a superpower, and then assume it is the same for China going forward.