In a nutshell, our statistics around employment, inflation, wages, and more are oriented toward averages instead of medians, and based on assumptions that applied in the era of full-time work more so than the current gig economy. If you look at how the bottom half of the country is doing - the half that swung toward Trump - it's a steadily worsening picture. No wonder the MAGA slogan held such appeal among people who remember living better a couple of decades ago. And no wonder Democrats' optimistic message that everything is already getting better got no traction, and indeed offended people.
What the article author fails to present is how it's different now when compared to other stable economic periods. If we're undercounting now, were we undercounting then? And is it worse now?
Without that data, all we can state is that there are more people un/underemployed than the (Edit: U-3) .gov numbers. We can't claim conclusively that there are more un/underemployed than, for example, 2007, right before the GFC. If it's not worse than 2007, then it changes the conclusion -- people are more pissed now because of...social media, inflation, political teams, cultural wars, inflation, the piece of straw, ???.
Edit: Can't we compare U-5 or U-6 to previous periods?
"U-6, total unemployed, plus all marginally attached workers, plus total employed part time for economic reasons, as a percent of the civilian labor force plus all marginally attached workers."