Fact of the matter is, it had to come from him.
For absolute certainty, of course, for a rational assessment, not at all. Remember Barry Bonds and Roger Clemens have never failed a drug test either.
As you said yourself when Greg LeMond became sceptical, so did you. When I heard his teammates turn on him, that was when I was certain beyond a reasonable doubt. If you need to be spoon-fed by direct admission of guilt, then you are frankly just a morally and intellectually immature person, because life is seldom that neatly resolved whether in our everyday lives or public scandals.
And beyond the cheating, look at the arrogant, egotistical personality type the were roaring in defence of. This is a guy who probably dumped his wife, the one who stood by him through cancer and gave him three kids, for a celebrity relationship (Sheryl Crow). Who floated running for governor. And managed to turn the admission of guilt, which is the bare minimum standard of morality and redemption, into another attention-grabbing spectacle. What a "legend".