A few decades ago, at what was arguably the peak of the US middle class, relatively few adult married women worked outside the home. But of course, they did work. It's just that they didn't get paid.
Or rather, they got paid through sharing their husbands' income. What's happened since is that both partners often work, but aggregate inflation-adjusted income (aka household income) has stagnated. Maybe even declined.
Anyway, the point is that, a few decades ago, maybe 30%-40% of workers earned nothing. So to get meaningful results, you need to look at aggregate household income over time. Including both partners, and children. And you also need to net out childcare expenses.
Also, I note that TFA relies on W2 data. So what about consultants, whose income is reported on 1099s? But maybe they're a minor factor, and can be ignored.
And finally, it's my impression that executive incomes are typically capped at a $million or so. With the rest paid as stock options and such.