The argument is that SCOTUS specifically mentioned auto tailpipes(a mobil source), but SCOTUS didn't mention stationary sources.
EPA says CO2 is CO2, whether it comes out of a tailpipe or power plant exhaust stack. And, SCOTUS didn't specifically exclude stationary sources in their 2007 decision.
I don’t see how they can differentiate between the two since they’ve already accepted that CO2 is a pollution source from whatever source but we shall see.