Earlier quoted context omitted.
I don’t think you understand how capital gains work... If you invest a dollar today and get ten dollars tomorrow you’ve gained 9 dollars. When were you taxed for that?
I think his point was the initial dollar you invested was at some point subject to income tax to get it.
If I invest $100 (post-tax money) and sell the investment at $1000, I do not pay taxes on that $100. There is no double taxing. I do pay taxes on the $900 gain.