The (now gray) comment talks about being double taxed. That is, taxed twice for the same money.

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.

The double taxation that some people talk about is actually corporate profits being taxed, and then that money being further taxed as capital gains / dividends - but I don't think that 's a fair criticism either.

The problem is the gain may exist in nominal terms only. It’s possible to make a 100% nominal gain, which will be taxed, even if in inflation adjusted terms you lost 90% of the actual value in the process.

Yes, but the capital gains tax doesn't apply to that dollar (it's right in the name).

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.

That’s a bad point, because the first dollar won’t be subject to cap gains, so there is no double taxation happening.