Suppose Landis was telling the truth when he claimed he had naturally high testosterone levels, and suppose this characteristic gave him a competitive edge. Would that render his amazing accomplishment a "mirage"? Obviously not, unless an athlete's innate talent also gives him an unfair advantage and makes him a fake.
To see how untenable the natural/artificial distinction is, consider altitude tents and rooms, which simulate the low-oxygen environment of high elevations in an attempt to improve endurance by spurring the production of red blood cells. The World Anti-Doping Association is considering a ban on this widely used technique, which its ethics committee deems contrary to "the spirit of sport."
If performance-enhancing drugs violate "the spirit of sport," it's hard to see why performance-enhancing rooms don't. If anything, they're even less natural than steroids. Yet banning high-altitude simulations arguably would make contests less fair, giving an advantage to athletes who happen to live at high elevations or can afford to move there.
Athletes use all sorts of technology to improve their fitness and performance, ranging from multivitamins to weight machines, and they are properly judged by how well they use them. Instead of arbitrarily prohibiting certain techniques, why not level the playing field by repealing the prohibitions?