I think it is remarkable that, as a historian, Mr. Tainter is unaware of the fact that 70% of the people in pre-industrial Europe were farmers. Which means that they could afford to have three times as many people in non-farm occupations than the 90% figure he brings up. All that with very little of what we consider modern technology.
Furthermore, the conquests of Rome certainly weren’t made to extract stored solar energy. Grain and food is produced annually and Rome didn’t commit genocides most of the time. The population of the conquered territories was still there and had to be fed with food grown annually on a fixed amount of soil, rain and sunshine.
The energy density of fossil fuels also pales in comparison to nuclear fuels (whose density is on the order of 10 million times higher). And while an electric tractor or combine harvester is certainly less convenient than conventional ones, they are perfectly feasible. Without putting to much thought into it, you could always hook up a cable on a powerline along the side of a field and spin it off a reel while driving down the field in order to plow it and spinning it back on, on the way back. Move the cable a couple meters to the side as you turn and do it again. You wouldn’t even need a battery. It’s quite a hassle, of course, but much less than a pair of oxen or horses.
I’m sorry, I’d like to be less dismissive of what he said, but I have no choice since I’ve committed myself to reality.