Take the evidence cited about how NEETs make up 10% of young people. However, the report itself offers one explanation for the numbers. "More than two-thirds of NEETs, in particular young women, are not actively looking for work. The NEET gender gap is larger in Japan than in most other OECD countries as many women in their late 20s withdraw from the labour force to care for children." People leaving the workforce to pursue homemaking and childrearing shouldn't be a societal problem if it's done voluntarily.
On the other hand, in the US, unemployment among 16-19 y/o's is around 14% and among 20-24 y/o's is around 7%. These populations are actively seeking work and unable to obtain it (and not homemaking), meaning that the number of "American NEETs" is likely higher than these numbers. The author seems to suggest that the US is downstream from Japan and that the epidemic of NEET-ism will only get worse, but if the current numbers are any indication, America's NEET problem is far worse than Japan's.
But the differences in unemployment between the two countries seem to be something more systemic. The total unemployment rate in the US has been roughly double Japan's for the past decade. And of course, it is natural that younger populations are hit harder by scarcity of jobs because most of them have little work experience. The author seems to suggest that shifting cultural trends are causing younger people to increasingly choose a life of unemployment. But past generations have had this same class of youth, be it the beatniks of the 60s, hippies of the 70s, or "Jay and Silent Bob" suburbanites of the 80s and 90s. The primary shift is in how this class is presenting itself culturally. Many of these people are simply embracing a lifestyle of joblessness to feel content in the lack of opportunity being presented.
The author cites a lack of human labor as a source of Japan's societal woes, but this is a leading contributor to why the country has what the author calls an "envious" 2.5% unemployment rate. Importing labor en masse from the third world will inevitably limit opportunities for citizens, especially young adults who will be competing for entry level jobs. Furthermore, lower class wages decline, increasing income disparity. The data supports this, Japan has a significantly lower Gini-coefficient (37.9) than the US (47.0). Indeed, keeping migration into the country low is not a source of problems, but instead giving Japan the upper hand on important measures of economic well being.