The article presents that caveat straightforwardly:
The popular view of America's upper class is that of an
ossified aristocracy. But research from the National
Bureau of Economic Research shakes up this view, at
least among America's richest individuals.
And elaborates:
Over the last 30 years, income inequality has grown and
intergenerational mobility has decreased. The rich are
getting richer and drawing up the ladder to the upper
class with them.
The story differs for the members of the Forbes 400 List.
So I'd say you're in violent agreement with the author.
The problem is that on one hand the try to talk about "America's upper class", on the other hand they restrict themselves to looking at members of the Forbes 400.
In other words: They're seemingly trying to make the story sound more relevant than it is, by framing it as if they have something to say about "America's upper class", when the data they look at consists only of a tiny subset of extreme outliers.
Bill Gate entered the upper class based on birth. His own efforts brought him yet higher. But Bill Gates proves exactly nothing about how hard working people move into the upper class, because he never ever did so himself.
