The only part that's a myth is that it's only a single decade. Japan's GDP has been flat for over 30 years now. USA's has tripled during the same time period.
Japan's population in 1980 was 116.8 million. 126.3 million in 2020. That's an 8% increase.
The USA experienced nearly a 50% increase over that same period.
Meanwhile, Japan's GDP went from 1.105 trillion to 5.04 trillion, all while reporting greater well-being among the population.
It's a myth.