Jonathan Adler, at the Volokh Conspiracy, thinks that Mass v. EPA does not dispose of standing in this case. He relies on the fact that a procedural injury was at issue in EPA and also that redressability is much murkier in AEP. Part of the problem is that the majority opinion in EPA is so obtuse. Massachusetts asserted a proprietary injury (loss of its coastal land), a procedural injury under the Clean Air Act, and a quasi-sovereign injury. The Court did not neatly unbundle these injuries, but lumped them all together and opined that states deserve "special solicitude." That can't be true with respect to proprietary injuries (unless Lujan was implicitly overruled, and the Court gave no suggestion that Lujan was infirm). The "special solicitude" must apply either when a state has a procedural injury or is asserting a quasi-sovereign interest (or both). Yet, the Court in EPA gave no reason why states, rather than other litigants, should receive "special solicitude" in cases of procedural injury. In any case, procedural injury is not at issue in AEP. That leaves the quasi-sovereign interest as the place for "special solicitude." While it's true that redressability is much more problematic in AEP, if the redressability standard is "any little bit helps" (which is a ridiculous standard, to be sure) the AEP plaintiffs meet this low requirement.
But the real reasons why I think standing will be found in AEP is that (1) it offers an opporunity to clarify EPA by ruling out any possibility that states have relaxed standing rules when they assert proprietary injuries or that EPA displaced Lujan more generally, and (2) it's necessary to find standing in order to rule that the Clean Air Act both displaces federal common law and preempts state nuisance law as to these claims.