... The whole point about claiming islands is that they include an EEZ (exclusive economic zone). That's how you claim a 'whole sea'.
Of course, the real reason people argue over islands because it gets idiots riled up (Abe and successors depend on the nationalist vote, Xi depends on the nationalist faction, etc) and because geography in the far east is a mess of contradictions, so it's easy to make a plausible claim.
Foreign people getting excited over it is like a whole new meta level of stupidity.
So trying to bring Japan into this is just ridiculous.