For me at least, much like vacuum cleaners and hoovers, 'Mountain Biking' is really just a brand name which has entered the common vernacular to describe a type of cycling.
The main difference between the mindset of the Rough Stuff Fellowship and the more modern Mountain Biker is the perceived importance of actually riding the bike on one hand, or simply just having the bike with you on the other. In many cases for the RSF it's more about the challenge of the terrain than necessarily being able to ride it.
Rough stuffers and pass stormers would/will think nothing of carrying a bike for 90% of an unrideable (and preferably snowbound) route. In fact it appeals to the expeditionist within them.
Whereas the mindset of a mountain biker is to ride as much as possible no matter how steep and technical, and as fast as possible.
In California in the 1970's, the addition of a range of gears to old bikes and the influence of road and then off-road competitions led to the development of what became the 'Mountain Bike'.
Mountain bike is just the name that stuck. I suppose it could just as easily have been the klunker, the fat tyre, the ballooner, the all terrain bike etc etc.
This certainly shouldn't in any way undermine those very significant developments in the 70's which paved the path to the modern mountain bike. It's nice to be able to put things into a wider context too though.