Jeet Kune Do: Not intended to be a first art?

A friend of mine once told me that he once read somewhere that Jeet Kune Do was never intended to be an art to start off with, it was meant to improve upon another art. If a Jeet Kune Do guy with two years of experience were to go up against, say, a kenpo guy with one year of experience, nine times out of ten, the kenpo guy would win. Any thoughts on this?