I think it's just sloppy terminology on the authors part, using "clean energy" to refer to wind/solar/whatever new method that has much different pricing charac

I think it's just sloppy terminology on the authors part, using "clean energy" to refer to wind/solar/whatever new method that has much different pricing characteristics to stuff that was making up a large percentage of the grid 20 years ago (like nuclear). Like how in programming code written yesterday can be called "legacy code".