Those are definitelly Hidden. The question is: can they enslave any unit or do they have a set of units, that they always use. Really climatic would be if they use only units from races that are present on the planet the invasion is fought on.
I am betting and hoping that the enthralled can be units of any faction, npc or otherwise. It looks like they just attached mods to units here (Enthrall mods that can't be removed). Would be kinda like that trope in mass effect where you fight reaper assimilated races: Fighting the populations of cities that got captured and enthralled by the voidbringers. This will be so cool if that's the case. If it's just kir'ko, that'd kinda suck.