How did Japan come to have a unified culture/national identity, across the archipelago? (Or does it actually not?) In another sub, someone claimed Japan is "the civilization that has most rebellions in human history and perhaps the most vibrant hermit culture in the pre-industrial world." Accurate?

The redditor was trying to refute the idea that Japanese culture is exceptionally collectivist. Speaking of, what do our Japan experts make of the idea that Japanese culture's supposed extreme emphasis on the group is the result of early communities being based on rice farming? I.e., rice farming can only be done in a group, so if you get kicked out of the group, you're in trouble.

Edit: "Supposed" bolded for emphasis.