The term "Darkfallen" only came into common use with the release of the new book. It showed up in ICC, but wasn't anything focal and people just used San'layn more than Darkfallen because it was brought up more.
The community as a whole, including more casuals - only the more hardcore fans even know of the actual term - knows what a Dark Ranger is. It is just easier to call it a "Dark Ranger" skin than a "Darkfallen" skin because your average player just recognizes it instantly.
People are nitpicking over the term when it doesn't actually matter all that much.