Cheers to David Currie for pointing out that while the WebSphere Integration Developer user interface allows you to specify an event sequencing qualifier inside a mediation module, if you attempt to deploy the solution to an WebSphere Enterprise Service Bus Server, you’ll get exceptions.

Interestingly enough, if you deploy the same Mediation Module to WebSphere Process Server, then it will work because Process Server is a superset of WESB and has the required runtime.

I highly doubt that this was an intentional UI design decision in WID.