>> I'm always a bit weary, when people rely on physics or rigid math to derive economic conclusions. In this case the author assumes causation between energy production/consumption and economic growth.

No, the author assumes energy as a prerequisite for economic activity - not a cause. That much is true. There are no significant economic activities that do not depend on energy. The author even suggests that more non-energy activity will be required to sustain economic growth and has a hard time with that.