The post addresses this question in detail. In brief summary, extracting "green" energy from the grid (most mining occurs on the grid) means that the energy available on the grid is more likely to be from CO2-generating sources. The amount of demand for that grid power is partially non-negotiable (things like street lamps, life support equipment in hospitals, etc) and partially negotiable (if the cost goes up enough people might stop using tv or indoor lighting, I guess?), but ultimately if demand spikes there will just be more coal and gas getting burned to provide power on the grid.
Also, the green power generation requires the manufacturing and distribution of things like solar panels, which is not currently CO2-neutral and has other environmental impacts.
There are also theoretical (if not real) limits to how much you can actually expand grid capacity. Mining already even in conservative estimates consumes a lot of power, and unless PoS is fully rolled out it will continue to consume more and more, which will put increasing strain on local power grids.