None of what you quoted here offers a different origin of employer-linked health insurance, and it avoids even mentioning that they were excluded from taxation. The fact that they weren't included in wage caps is just another benefit granted to employers.

> if anything, employer-based health insurance strengthened the unions' ability to negotiate for their workers.

In what way does having more that needs to be negotiated strengthen unions? You've heard the punchline without hearing the joke: why were unions prevented from negotiating pension and insurance benefits before 1949?

If you reversed the original sin of sheltering wages paid in the form of health insurance from taxes, you'd see stronger unions the next day. The biggest battle fought during the ACA war was about the possible taxation of "Cadillac" health plans, and ACA lost.

edit:

Strike wave of 1945–1946

https://en.wikipedia.org/wiki/Strike_wave_of_1945%E2%80%9319...

Taft-Hartley Act

https://en.wikipedia.org/wiki/Taft%E2%80%93Hartley_Act

Exclusive Union Control of Pension Funds: Taft-Hartley's Ill-Considered Prohibition [PDF]

https://scholarship.law.upenn.edu/cgi/viewcontent.cgi?articl...