Roberts appears to question EPA's side, so I'll wait for the verdict.
I don't think it is quite that simple. I think the three bimbos have accidentally stumbled onto the right side of the issue even if not for the right reason. Several liberal states brought the suit, trying to get the district courts to impose emissions regulations on the utility companies in question by fiat. The suit should be thrown out based on the simple fact that the courts should not be directing regulation policies like that -- regardless of whether or not the EPA has the authority to regulate CO2 and other "greenhouse gasses". So while the three bimbos' primary reason seems to be that the EPA is the authority here rather than the courts, they are still getting to the right answer even if their reasons are flawed. (This may be one of the few times when it is better to just state the answer without "showing your work" in their case.)
Roberts appears more focused on the really core issue that the courts should not be creating policy or issuing regulations on behalf of the states against the utility companies.