Air barrier below conditioned attic with unvented hybrid roof? – GreenBuildingAdvisor

My understanding is that when building a conditioned attic below an unvented hybrid roof assembly, it is not necessary to install an air barrier on the underside of the roof assembly, directly below the fluffy insulation (as you would when building an unvented cathedral ceilings). For example, see the attached detail, from Martin’s old article An Unvented Superinsulated Roof. In this scenario, is it necessary to exhaustively air-seal the attic floor (as you would when building a conventional vented attic) in order to atmospherically separate the conditioned attic from the conditioned space below?

If the answer is “yes”: why does the fluffy insulation need to be separated from the main conditioned space, but not from the conditioned attic space?