Of course, you are correct in principle.
However, Japan's claims do not interfere with other nations' claims in that particular case. China has territorial disputes with Taiwan, Vietnam, Malaysia, Indonesia, the Philippines, Brunei, and Japan and creates faits accomplis in the South China Sea by occupying and concreting over reefs and shoals. It was China's expansionism that started this wave of land grabbing (Vietnam, too, has begun to fortify shoals).
Also, I assume that Japan would accept the decisions of international courts in case of territorial conflicts.