From the article:
Using U.S. Census Bureau data, the institute also found 57% of working-age Americans — more than 100 million people — don't have retirement assets in an employer-sponsored defined contribution plan, pension plan or individual retirement account.
The NIRS report noted that even among retirement account savers, the typical worker had a balance of just $40,000.
To the first quote, what is the definition of "working age?" I didn't see it in the article. Google gives it as 15-64. Even if it's, say, 18-64 then there's a lot of people close to either endpoint with justifiable explanations. People under 25 are often in school without ever having much of a job. A lot of people 50+ probably still were in a one-income household, where one person works (and has the retirement accounts) and the other raises the kids.
I really don't like the methodology of giving statistics about all types of people of all ages and backgrounds (how about new immigrants, how much of a 401k should they have)? This brings us to the second quoted portion. There are a lot of millenials. Someone under 30 years old with $40,000 in a retirement account is actually doing pretty well. And, since it's now "retirement account savers," people 55+ often had pensions. Anecdote alert, but I have a buddy who recently retired with $16,000 in a checking account. How? He made it to 20 years as a state government employee and gets a $50k annual pension. He and anyone in a similar situation would probably actually show up in the 0 category if we're going by retirement account savers.
Then, as others have mentioned, people can have multiple retirement accounts. Using their methodology, would someone with three $40k accounts show up as one $120k, or as three $40k data points?
There are definitely people out there who will have a tough time in retirement. And there are others who will be ok. But statistics like these do not really shed any light on the relative frequency of either.