Eh, it's kickboxing. I don't think anyone really cares about doping in kickboxing. Let him eat horses.
To me, this is part of the more interesting question: where does Overeem sit on greatest MMA HWs of all time? In my opinion, he's definitely somewhere in the top 10, and I could see an argument for him being in the top 5. Excellent W/L ratio, quality of opposition, and finishing %. However, you could say that he had a habit of falling apart in big moments, and he didn't do himself any favors with his PED suspension in 2012.