Wayne, I don't think that diagram is anything more than a "block" diagram, not intended to be taken literally.
I can't say for sure, but there's so much missing from it, I think it's used as a general guide to understand the rest of the system, and is there only for that purpose - since they intended the cluster to be a "module suitable for replacement only".

It's stupid, IMO, but I really do think that is the case.