Most of competing claim come from economic zone of UNCLOS agreement But China claim predate UNCLOS and tacitly reconqnized by the US since they are the one that transported the Nationalist troop to Taiping island after Japanese surrender
It is no more ridiculous than Britain claim over Falkland or French claim over New Caledonia
At least the SCS is close to China and has been the fishing ground of Chinese fisherman for generation. What give the French the right to colonize half world away They cite Terra Nullus principle So yes China can applied the same Terra Nulus principle. What is good for the geese is good for the gander
BTW UNCLOS does not has jurisdiction on national boundary It still have to be decided by the relevant countries
China settle 13 out of 15 boundary dispute with the neighboring countries that show that china can settle border peacefully