Yes, and the dissent spells that out clearly. An excerpt from the dissent:

> The majority says it is simply “not plausible” that Congress enabled EPA to regulate power plants’ emissions through generation shift- ing. Ante, at 31. But that is just what Congress did when it broadly authorized EPA in Section 111 to select the “best system of emission reduction” for power plants. §7411(a)(1). The “best system” full stop—no ifs, ands, or buts of any kind relevant here.

They were given broad power by congress explicitly. If congress had wanted them to reign in that power or not choose the best system, then they could pull it back at any time. They did not. The court decided that the previous congress was full of idiots and could not fathom future technologies for emission reduction and therefore is moot.