The US middle class is not the same, 1975 vs 2022.

Show me the demographics of 1975 for the US middle class and lower middle class. Then show me the same demo for 2022.

Next show me the wealth and income figures for each demographic, 1975 vs 2022.

Hint: the US middle class is no longer entirely white; and the white people that used to dominate the US middle class have moved up. Meanwhile first and second generation Hispanic Americans have moved into the US middle class, coming from largely impoverished backgrounds and third world countries. Does that document account for any of that? Of course not.

You can trivially see this in the Fed's demographic data for wealth and income (white household income and wealth figures are well above the median figure for example; which was not true in 1975, back then they were close to one and the same), and by comparing present demographics vs past demographics.

Hispanics were 4-5% of the population in 1975.

If the white demographic moves up, and formerly poor Hispanics move into and take over the US middle class (which is exactly what's occurring), the document you linked to pretends nothing changed, there was no improvement. There was in fact vast improvement for the actual people in question.