I don't know if he doped, or not. I can understand why he would say F it though. If he is innocent, this is just a giant witch hunt so why waste the time, energy and money to combat it for years and years and years. He was tested umpteen times 'til Sunday at the time, and his results were clean.
He could always employ the OJ defense, that the samples have been tampered with especially after such a long time. He concluded it isn't worth the grief. He is retired from the sport so the lifetime ban is sort of a joke.
From what I have read, it is not exactly a given the international committee will back the allegations of the American one. We'll see.