In a culturally homogeneous and tightly-knit society, in which strong traditional norms covering all aspects of life are taken with great seriousness and individuals normally don't spend much time in isolation and anonymity

I'm absolutely not an expert (I've never even been to Japan), but this really doesn't sound like modern Japan to me, at least based on what I've absorbed from pop culture.

To what extent the contemporary Japan has these characteristics, I don't know, but I'm pretty sure that it has them in a greater measure than, say, the U.S.

More so than the US, sure - but the US is an anomaly when it comes to cultural heterogeneity. Lots of countries with western-style legal systems have a similar level of cultural homogeneity to Japan, but don't see this same phenomenon.