"funded by my already taxed earnings"
Why did you even try that? Blew your whole argument.
why do you disagree? Most people working a job do have taxes taken out. That's why you get a "return" when the IRS realizes they took too much or you provide other means to deduct your taxes.
The comment attempts to suggest that something is being taxed twice. It is not. The original income that funds some investment is taxed. After that, only further income is taxed. The existing principle is not.
Nothing that was taxed was "already taxed".