There is no way Diego thinks his stand up is as good as GSP. That is why he knows and has said that he isnt ready to fight him. Diego is a very smart fighter, he knows that if he fights GSP that he will probably lose because he knows GSP is better than him. I dont think he was saying his stand up is as good as GSP in that interview, more so that it is better than Matt hughes, which it is. Diego is a training machine, the only things he needs to work on for a fight with Hughes is ground game and takedown defence cause I think Hughes will be smart enough to not keep this one standing especially after his last fight and watching Diegos last fight.
And even if Matt does get on top of him, remember Diego is crazy slippery in a Penn sort of way. He is very good at taking the back of his opponent from the bottom, not as good as Penn but you know what I mean. I really do see Diego winning this fight if they can get this too happen. He definetly has all the tools and he has never been proved wrong before.