I think you should select all the drum MIDI- and audio tracks AND the bus, and select bus as the source.
But I still think there should be no need to use a bus in order to bounce five tracks to one. I doubt that feature had been changed so much in X-series. I've bounced countless times my whole project to one stereo track inside the project. Unfortunately my DAW isn't in working order to be able to brush up my memory, just moved to a new place.
Anyway, selecting both the MIDI- and audio tracks is necessary.