That defines “middle-income” as 2/3 - 2 times median income, which is such a fake definition, and the groups all got wealthier since the 70’s. And even with that, the article is agreeing with me, not contradicting me:

“The recent stability in the share of adults living in middle-income households marks a shift from a decades-long downward trend. From 1971 to 2011, the share of adults in the middle class fell by 10 percentage points. But that shift was not all down the economic ladder. Indeed, the increase in the share of adults who are upper income was greater than the increase in the share who are lower income over that period, a sign of economic progress overall.”

It looks like they’re also cherry-picking a year 2000 baseline in some of their charts.