That's assuming an equal amount of carbon is produced by a given amount of extracted material, regardless of how it is used or processed. That seems an incorrect assumption completely disregards potential processing improvements that would reduce it.
This is why you need a true carbon tax if you want to go down this road, as it would make you pay for the actual carbon you produce.
Applying this uniformly to all manufacturers around the world, along with users of final products (like gasoline) would be very difficult though.
Taxing imports causes mass famine? That sounds bad! I wonder if there is a level of tariffs and such where the post you replied to could be effective without accidentally making everyone starve and overthrow their government. Do you have any info about the tipping point there?
This is why you need a true carbon tax if you want to go down this road, as it would make you pay for the actual carbon you produce.
Applying this uniformly to all manufacturers around the world, along with users of final products (like gasoline) would be very difficult though.