If you pay taxes with gold coins at their face value instead of paper dollars (i.e. taxes total being equal coins face value total), you effectively end up paying more than you would pay if you previously sold your coins at their market price and paid the original tax plus the tax due to the commodity value of your coins.
And who would be stupid enough to do that?
People are stupid, but that not that stupid. Most of them can count.
This doesn't render payment of taxes with gold coins at their face value either impossible or eligible for taxing the commodity value of them (which was your point). You can't pay an income tax on an income that you didn't get (that was my point)...