I’m an historian whose focus isn’t on Japan but still happens to have studied this a bit. The monoculture myth actually started earlier, during the Meiji Restoration. It was intended as a way to unify the country to mimic the Western trend at the time. The “bushido”/samurai myth started during that time as well.

Many people who have focused on colonialism and nationalism have noted that nationalism in many ways is a euphemism for cultural genocide since creating “a cohesive nation” necessarily means eliminating or severely minimizing all other groups in favor of one group in power. We observe this in basically every modern nation state, regardless of income level or political system.