I can’t tell from your comment but I’m not sure this is what you expect. This is not people who have never had a job. This is a (now politically motivated) estimate of the number of employable people. Subtle difference.
Whether it's generally accurate is another question.
But a Google Scholar search will (along with numerous false hits) turn up multiple discussions:
https://scholar.google.com/scholar?hl=en&as_sdt=0%2C39&q=une...
I've chosen Scholar rather than a general Web (or literature) search as the frequency of this accusation online or in the general press is so common as to be a trope, though you can repeat my query in either.
My own read is that unemployment is at best an incomplete metric for reasons expressed in the Brookings article: it fails to consider wages or compensation, and a large number of low-wage, low-skill, low-opportunity jobs really isn't commensurate with a strong labour sector.
People without college degrees get disability and stop seeking employment, making the economy worse off but making the employment statistics look better.