I would argue (because I like to argue :)) that it's both. Those elite RBs have elite traits that other RBs don't. If you put an average RB behind an elite O-line, you will certainly get an improvement by the RB, but not elite. A good O-line can't make a slow RB fast. Once the RB gets past the line of scrimmage, it's out of the O-line's hands. If you put an elite RB behind a good O-line, do you get the same results as a good RB behind an elite O-line? I don't know. But I tend to think that elite RB + good OL > good RB + elite OL.