The efficiency for burning fuels is, by definition, the ratio of the useful energy obtained divided by the energy contained in the fuel. We doubt that Stevens was responsible for the misleading headlines.
Efficiency does play a role, of course.
But that's still not what Stevens is driving at. For nearly a century and a half, fuels with high amounts of carbon have progressively been replaced by those with less. Aside from unwanted minerals that degrade the quality, coal is all carbon. Petroleum has roughly two atoms of hydrogen for every atom of carbon. Natural gas contains four atoms of hydrogen for every one atom of carbon.