At the risk of stepping into the fire, let me try to clarify. What Joe is telling you is not what has been determined by established players. Established players, at first, interpretted the wording in the book just as you have and where using horde/cult grunts to bring Apostle elites. It was Thom himself who said that this was not correct. So it is Thom's interpretation that states the horde/cult and Apostle lists need to be able to stand alone even when played in the same force list.
Your analogy about never being able to play a Bauhaus Duke for lack of grunt options would be correct if the corps and the DL followed the same structure. But, as Joe previously stated, they have different criterias. This was determined by Thom and that is why Joe said that you just have to trust him. Joe, as part of the FAQ team, is just acting as a conduit from Thom