If Grunge is just a movement and not a stylistic entity, then the matter is settled by answering the following question:
Did Nirvana and in particular Cobain really identify with the grunge movement? 'Cause a nuraber of fans claim that Cobain never really wanted anything to do with grunge and didn't see Nirvana as a part of it; that the only reason Nirvana was grouped with grunge was because they happened to be from Seattle; that the term "grunge", in respect to Nirvana, was just used as a promotional vehicle and for commercial enRAB, in other worRAB, because they could; and so forth.
Not saying that is correct, just what some do claim.