cannot regulate CO2 emissions, it just says that the EPA wasn't authorized by congress to use the "generation shifting" approach it adopted in 2015 to address limiting emission under the Clean Air Act.

EPA can still use the Clear Air Act to regulate emissions - but the novel approach taken under the 2015 Clean Power Plan to require current power producers to use "generation shifting" to shift power generation from one type of production (i.e. coal or natural gas) to another type of production (i.e. natural gas or renewables) wasn't within the authorization congress gave the EPA in the Clean Air Act.

It doesn't come close to eliminating the regulatory state or eliminating the ability of agencies to enact regulations - its saying that this regulation wasn't within what congress had authorized the agency to do.