if you dont think GSP or many of the top guys back then werent using some sort of performance enhancer then you are just really naive. These guys had no reason not to use one as the testing was not existent and even with the early stages of usada you could still bypass it.
This is like asking if we think pride fighters were clean, like come on bro, if you fought in pride and you were clean then you were doing it wrong, same thing in the the ufc before the testing.