The author of the new Costello book, Anthony M. DeStefano, claims Genovese is Sicilian. He leaves no footnotes or reference to refute all of the other claims that Vito was Neapolitan, so maybe he sees it the way you referred to, that it's all southern Italy, anyway, no point in splitting hairs.

In the Scorsese documentary "Italianamerican", where he interviews his parents (who've appeared in a lot of his movies), I believe his father says something bout how in Sicily it's all about what village you're from. Sicilians don't even care if you're Sicilian. If you're not from their village, they're suspicious of you, or something like that. It was a few years ago that I watched it, but he seemed to say something to this effect. The Scorseses emigrated a long time ago, though. A lot had to have changed in Sicily since the end of World War 2 and Mussolini, although the mafia and their provincial standards obviously remained influential.