Much-disputed terms like "film noir" or (another favorite academic bone of contention) "the Enlightenment" are necessarily a little loose and shifty because they are simply language of convenience for conveying an overall impression of a complex phenomenon to newcomers or students. They are "coarse-grained" terms. At a finer level of granularity, the concept often simply disappears, hence articles like "Film Noir Doesn't Exist: A Lacanian Topology," or a number of academic books that say the Enlightenment doesn't exist, or various art movements don't exist, etc. The people who put forward these theories are always deep into the subject themselves, working at a fine level of grain, and because they personally don't need the coarse-grained terms anymore, they think no one else should use them either, or should only use them in ultra-precise ways, which is ridiculous.
I have often told my history students that there is a 1.0 version of history, a 2.0, and so on, and that from the standpoint of the 4.0 version (graduate school level, say), a lot of the 1.0 version looks flat-out wrong. But when you're teaching high school students, you've got to start with the 1.0. You can't go the highest level of complexity right off the bat! No one will follow you.
William Ahearn is a touchy guy (I can tell you from personal experience) and rides his hobby-horse hard, which is that because he believes he has a more advanced 4.0 understanding of noir (maybe he does, maybe he doesn't), everyone else should have exactly that understanding too, and all 1.0, 2.0, and 3.0 understandings should be discarded. Maintaining this sort of position is what gets one labeled as cranky.
I will follow up with a post giving more detail on the concept of granularity.