World War II was a much bigger reason and a cause for this. There is a problem with correlation vs causation in the article’s logic. I’d argue the unions were a bi-product of a historically rare moment in time vs cause of equality. The market bust and global world war increased taxation, demand for workers, and value of each worker in the early 40s. Much more importantly, for a few decades after WWII all other global world powers were rebuilding after being leveled by the war. The US instead remained as the last productive developed nation standing for decades and enjoyed likely huge margins as a result of both lack of competition and demand for supplies by the rest of the developed world. Those margins and the unscathed living conditions, also turned the US into a magnet for technical and scientific talent from everywhere else.
The more competition recovered in Japan and elsewhere, the less bargaining power and surplus could be captured by Unions. The Union-driven higher costs of manufacturing also drove outsourcing, off-shoring, and did not lead to higher quality output, (if Unions were producing higher quality that would have kept Japanese-manufactured cars from dominating US car sales shortly after).
Even further, exorbitant labor costs make it worthwhile for manufacturers to do capital investments in automation, and we are seeing the output of that.
The rise in middle class should probably be attributed to investments - in equities, bonds, house purchases. Growing up in Eastern Europe, where only housing investment was an option, and unions were abundant, the middle class that sprouted came from those who invested in property and business. Labor union power and wealth was too politically and corruption influenced to be a sustainable source of growth for the country.