> The few that I know for a fact come from exceptional money
The article is not about people "coming from exceptional money". And I bet a lot of the people you are assuming are "middle class" aren't actually middle class.
There is a really bad habit in this country of assuming that you can have money, but unless you are "movie star rich" then you are middle class. That's not the case.
A lot of people work for a living but are not middle class (nor are they CEOs of fortune 500 companies).
More specifically a lot of the people reading HN right now likely fall into the category the researchers talk about. If you work in tech there is a very, very good chance you are not middle class. Just because you don't think of yourself as rich doesn't make you middle class.
It's something that being in an echo chamber of tech that people easily forget.
Someone can go look up a more precise definition, but if middle class were the middle 50th of household incomes, then if you have a household income (not individual, but combined AGI) that is > $100k you are not in the middle-class.[1] That doesn't mean you have multiple yachts, or act privileged or have exceptional money. It simple means (by that definition) you aren't middle class.
And before someone replies with it, "Cost Of Living" arguments do not change this substantially at all. Look at the AGI tables below, many people who think of themselves as middle class, are not. And if you're on HN reading that post, with a US job in tech, there is a good chance that includes you.
[1] Income in the United States - https://en.wikipedia.org/wiki/Income_in_the_United_States