The author's data says the median net worth for 65–74 year-old households is about $410,000, but 56% of it is home equity. Exclude the house and the median net worth drops to $171,000 and the median financial assets (the part that could actually be converted to cash easily) are only about $115,000. At the 25th percentile, wealth excluding home equity is under $30,000 for every age bracket from 55 up. So basically the net worth number looks best for the people whose wealth is least spendable.
That calls into question the author's claim that "given their current net wealth, a solid majority of Americans can comfortably retire without Social Security." Back of the envelope math: safely drawing 4% on $115,000–$170,000 provides just $5,000–$7,000 a year versus a median SS retirement benefit in the low $20,000s.
For a typical retiree, SS is worth more than every financial asset they own combined. The author says that downsizing or reverse mortgages count as "doing fine" but that's just his opinion. Reverse mortgages are expensive and you lose your equity quickly, and downsizing in the current market basically means that you pay way more for way less.
From what I can tell, it's basically the top third who could do without SS comfortably. Not at all a "solid majority."