They further explain their approach to resolving claimed conflict between two pieces of legislation:
> And in approaching a claimed conflict, we come armed with the “stron[g] presum[ption]” that repeals by implication are “disfavored” and that “Congress will specifically address” preexisting law when it wishes to suspend its normal operations in a later statute.
And specifically for the Arbitrage Act, because this is apparently a tactic that people keep trying:
> In many cases over many years, this Court has heard and rejected efforts to conjure conflicts between the Arbitration Act and other federal statutes. In fact, this Court has rejected every such effort to date […] Throughout, we have made clear that even a statute’s express provision for collective legal actions does not necessarily mean that it precludes “‘individual attempts at conciliation’” through arbitration […] And we’ve stressed that the absence of any specific statutory discussion of arbitration or class actions is an important and telling clue that Congress has not displaced the Arbitration Act.
So they're not saying "we can't decide," they're stating (I think): Congress can give employees more protection in this arena, but the laws as written are (1) not unconstitutional and (2) are not being interpreted incorrectly by lower courts.