They start measuring the performance stuff properly in junior reps, which is when the articles alleges he started using it. The baseline performance measures may have always been under the influence of PED's and certainly when they get even more into the detailed measurements in the 20s.
Seriously, if you believe he has taken these substances without significant outside assistance you obviously still believe in Santa Claus and the Tooth Fairy. Doping at the alleged level involves many other persons involved not just the athlete involved Ben Johnson and Lance Armstrong to name a few. It is much more widespread than just getting into the right gym and suddenly finding access to a cocktail of performance enhancing steroids.