It is the difference in culture. In big cities, Japan doesn’t tolerate public deviance. Police are visible in every block. They are very strict about weapons; you can’t bring a knife in public for no reason etc.

To his point - I would say, it's a bug factor BECAUSE on average their culture seems more safe. But it's not because it's monocultural. Bad "monoculture" is bad, good one is good, nothing complex there. Simplifying, but that's pretty much what is said

You're missing the more important word.

Yes

Diversity doesn't make places more dangerous (if i understand the stats). But humans are naturally tribal and fear those who look and act significantly different.