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 For nearly a century and a half, fuels with high amounts of carbon have progressively been replaced by those with less.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.
A large fraction of the energy comes from combustion of the hydrogen. Therefore, per unit of heat delivered, less carbon dioxide is produced, as any student of high-school chemistry will understand. Hydrogen, it is true, contains no carbon whatsoever. Burning hydrogen produces only common old water vapor. More energy, in fact, than will ever be released from burning the hydrogen.