This mission lists HK assassins, elite HKs and an elite probe droid in the Reserved Groups, among other things.

At a certain point, you deploy one of these three things depending on the number of boons in the heroes play area. In my case it will be an elite probe droid. There is no other mention of deploying the other two units.

Does this mean the HK droids are trapped in my Reserved Groups and I'm basically not allowed to use them in this mission?

Reserved groups can only enter play from events. If the event that deploys them does not happen or the event does not deploy them, they won't be deployed. (See Under Siege for another example.)

To answer your question in a more simple terms: yes.

~D

Thanks, just checking to make sure I didn't miss a rule that would add them to my open groups or something.

yeah, i was excited for that mission, then i realise the rebels have no boons. and i get a single probe droid, leaving my HKs stuck in the phantom zone.