The median household income in the United States is $79.9K. Assuming that a family of four can live on $50K (including taxes) in - most - locations, which is twice the poverty limit, they can theoretically save $30K a year in a mix of 401K, IRA, and general investment accounts.
This amount, if invested over thirty years with a 7.5% annual return (which is lower than what the S&P has historically returned by a fair amount), would give them a nest-egg of $3M. If they only manage to save half of that amount, they would still have $1.5M by retirement.
Now, this is a very feasible scenario for families in their twenties to late thirties, which is why many of the names mentioned in the article harp on the importance of investing early. It doesn't work nearly as well once you reach that point.
However, the opposite - not saving or investing, and having a large amount of consumer debt - leads to significantly worse outcomes.