I think the issue might be that while the US guarantees the integrity of Japan itself, the current NCA has a track record of equivocation and lack of decisiveness.
This might lead certain people to believe that ownership of the Senkaku / Diaoyu islands is sufficiently contentious to be kicked into a UN long grass debate, rather than trigger a full-scale confrontation, should China execute a Crimean-style land grab.
If the Chinese objective is effective control of those islands and the exploitation of resource underneath them, mission accomplished.
That said, I suspect they'd have a very difficult time with the Japanese submarine force, which is more than capable of a sea denial campaign.