"Whether Class Arbitration?" Decided by Courts, not Arbitrators

Recent Sixth Circuit holding summarized by Liz Kramer "even though a plurality of the Supreme Court concluded in Bazzle that classwide arbitration is a question for arbitrators, the Sixth Circuit held this week that it is a gateway question that is presumptively for judges.  Furthermore, in this case the parties had not clearly and explicitly authorized arbitrators to determine the availability of class action, so the default rule governed."  The "gateway" analysis the Supreme Court started years ago guides lower courts on a variety of issues but I wonder if it's good guidance.