Instead of tracking the carbon impact of manufacturing, they should track the energy input. Or just track the overall cost. As you said, it is hard to calculate manufacturing cost, and it depends on the energy source for factory.
Then can assign cost to the carbon emitted or saved, and then compare that to the overall cost.
How do cost and energy usage play into this though? Let's say mining, refining and all industrial processes are electrified or using green hydrogen one day, electricity is really cheap, what does it matter how much costs are when everything in carbon neutral?
Then can assign cost to the carbon emitted or saved, and then compare that to the overall cost.