Right, and sentiment tracks the measures of middle class wage growth, of poverty, etc. quite well. It tracks less applicable statistical tools, like GDP, less well. But then, I am guessing that author knew that and intentionally chose a statistic meaningless to the middle class to make his point.
And his point is a sound one, in any event, but it does nothing to show we are actually in this entrenched downward trend like he says, or like how the public generally feels right now. Wage stagnation is a major issue, but it's not the defining issue for the republic.