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.
There is no double taxation here.
There is a more nuanced question as to whether or not inflation should be incorporated into the calculations, but there are pros and cons there also.