Thats actually not the point they are making. They are saying that their income was taxed once when they made it, then after taking the risk of investing it in the market it is being taxed again if they earn a profit. It is still technically false but now that so many people see things like index funds as a sort of bank account it makes sense why they would feel they shouldnt be taxed again on it upon withdrawal.

Bank account interest is subject to income tax; brokerage accounts are only taxed on additional growth or dividends, and often at lower than income rates for stocks. So I'm confused why people would think that index fund income isn't taxed due to any similarity to bank accounts.

people make the complaint about divvies they've drawn in their own company that they've paid corp on which is a form of this conflation.

That's part of the deal of being a corporation. If you want pass-through taxation, that's what partnerships and LLCs are for. Of course, if you also want outside investors...

This is one of those instances where yes, of course it would be nice for you if you didn't have to give as much money to the government in taxes. But that's just not how it works.