Guys like Bonds, Sosa and McGwire, who have denied the use of steroids, were never that believable. While they denied the use of steroids, they have either sidestepped the issue, and/or said they were against steroids in defense of their own case. I've never heard any of them speak openly and candidly about their stance against steroids.
Frank, on the other hand, has been consistantly outspoken on the subject throughout his career. Either he's telling the truth, or he has predetermined an elaborate plan from the beginning of his career to speak out against steroids in order to cover up his own use. And based on the many ill advised comments he has made throughout his career, it's really hard to believe that Frank would be capable of doing so.
As the Devil's advocate, I can understand what you are getting at with this. And yes, there is the possiblity that Frank could have been lying the whole time. It's just not very likely, nor would it make a lot of sense.