This was hard for me to follow. I think it's targeted toward people steeped in "peak oil" discourse.
His first assumption seems to be that economic growth is primarily caused by fossil fuel consumption (which probably is largely the case for the last couple hundred years or so).
He postulates, based on data trends, we are nearing worldwide "peak oil demand," which will cause worldwide economic stagnation for the foreseeable future. This thinking is kinda of the reverse of how I normally think of it (economic growth drives oil demand), but I suppose it's valid if fossil fuels are consistently too expensive to extract more of (lowering demand).
My takeaway: without growth, capitalism becomes a zero-sum game and cannot function "properly," so this peak-oil-demand will result in world-wide economic collapse or probably something slower (a crumbling?).
However, his analysis states as a fact that renewables aren't as "productive" as fossil fuels, so won't be able to cause future growth, or at least growth at the same pace as the last couple hundred years. I'm not sure I agree with that because I've seen charts that show the levelized cost of renewable electricity production to actually be significantly lower than that of fossil fuels.