Per a new post by Jonathan Adler, Justice Stevens, in his 2009 dissent in Entergy Corp. v. Riverkeeper, Inc., made the very same mischaracterization of the EPA position in the 2001 case (Whitman v. American Trucking Ass’n):
In that case [Whitman v. American Trucking], the Court reviewed the EPA’s claim that §109 of the Clean Air Act (CAA) … authorized the Agency to consider implementation costs in setting ambient air quality standards.
So it seems plausible that Scalia made the mistake of relying on Stevens’s mischaracterization.
That, I emphasize, would be no excuse for the gaffe. Indeed, relying on Stevens for anything would be a serious mistake. But it does render even more ridiculous all the over-the-top frenzy from the Left.