The NMB didn't "sign off" on anything. You are confusing mediation with arbitration. The IBT was bound by a merger clause to enter BINDING ARBITRATION by a third party to settle unresolved issues in their JBCA. This was confirmed in court in 2019 and 2020. The pilots didn't even get a vote on the finished product!! Binding arbitration is certainly an option when entering federally mediated negotiations, but both parties must agree to it. That would be ill advised.