As I read it (but this is getting into some nitty-gritty agency-jurisdiction law I might misunderstand), the jurisdictional argument is about what "common carrier" status does for oversight. Telecommunications law gives the FCC exclusive authority to regulate common carriers, because they aren't quite normal market participants, but instead more like a regulated utility with special requirements that apply to them. So the FCC is tasked with drawing up those rules and overseeing them, and the FTC doesn't oversee them the way it would oversee other market participants.
Mobile data did not used to be classified as a common-carrier service, but was reclassified recently (3 weeks ago, in fact). The court found: 1. The fact that AT&T provides a common-carrier mobile service doesn't mean that it automatically is immune from FTC jurisdiction in any mobile-related case. Instead it needs to show that the specific conduct in dispute is related to its provision of common-carrier telecommunications services, and therefore exclusively within FCC jurisdiction; 2. The specific conduct at issue here happened before the reclassification as common carrier, so the FTC properly has jurisdiction.