It's an old story by now: The cyclists and their doctors and labs were on the leading edge of these doping techniques, and the regulators were playing catch-up. The tests were not adequate because they had to be specifically tuned to known substances, and the substances were being changed, modified and newly formulated all the time. With the regulatory tools that existed at the time, the statement "I never failed a test" seems less than compelling.
Where I come down is the same as baseball: If there's a major aspect of your sport that's hidden, shadowy and secretive, there's something wrong. Its purpose is to cheat. There's plenty of evidence that was the state that existed -- and may still exist to some extent -- in professional cycling. There's testimony that Armstrong was involved in that world. And if he wasn't doping -- and I agree the evidence if far from beyond a reasonable doubt standard -- he did noting to correct the situation, either.