Basically, he argues (perhaps my interpretation) through statistics that income equality is very significantly related to what is typically considered a good country to live in, which in contrast to many, does not include salary/GDP and such monetary stats but rather how long you live, infant mortality rates, social mobility, etcetera, where the Nordic countries and Japan outperform all other countries (more or less) and where the USA have it really difficult to compete against other Western/developed nations.

Thus, one can quite easily argue that regardless of the adjusted GDP growth, Japan has managed to develop in a much more healthy way compared to the USA, which may be because of the quickly rising income equality.