Never heard of Petey Green, but that was funny.
Extreme bodybuilders do all sorts of unhealthy things to their bodies that no doubt start to screw them up (like chronic steroid use). Dedicating one's life to the artificial enhancement and over development of muscles has little to do with health. So, while I'm sure these people have loads of info on how best to achieve this, what is healthy and good for the body is not their strong suit.
If you live a reasonable healthy lifestyle, then "detox" is not a real issue. However, if you spend your life destroying your body with alcohol, chemicals, steroids, drug use, etc, then I don't really have a hard time believing that it does, in fact, screw up your liver. I don't, however, believe that eating watermelon for a few days will repair the damage being done to it.
So, maybe there is some anecdotal "proof" that it works, sort of like there is anecdotal proof that "Metabo4000 Extreme Fat Loss Now!" works too, and we all know that is reliable that is.