If I had "proof" I wouldn't say things like, "In my opinion".
Metta went to a doctor, whose name he will not disclose and who is not affiliated with any NBA team or any disclosed surgical or treatment center, for a "new technique" purported to be similar to " Platelet Rich Plasma". It was recommended to him by Kobe (who got his recommendation from a cyclist who is now banned for doping) and A-Rod (also ejected for doping). His career was nearly finished before the "special treatments" but then it suddenly revived. When asked how he was playing like he was younger, he said it was because of "yoga". When asked about the "special treatments" he says it's something he cannot talk about.
The NBA doesn't test much - it tests less than almost all other professional sports. This is a guy who, in my opinion, is doping. Eventually he will be caught. Eventually.