The scaling and net energy argument is that it's relatively easy to have a surge when the overall share of renewables is relatively tiny. Much harder if you'd have to run all the extraction, manufacturing, transport etc that goes into the creation of renewable energy infrastructure on nothing but renewables. You might still run a profit (in terms of EROEI, or energy return on energy invested), but would likely end up with a much smaller surplus compared to a society that runs on fossil fuels, ie a less affluent society overall.
Greer's view is that a managed downscaling or decline to this kind of low-energy, sustainable society should've happened decades ago. And that at this point there's no way to avoid some sort of uncontrolled, disruptive collapse to a lower-energy level.