Evidence of what? That GMs purposively built teams to lose, or that teams purposively lost individual games, or something else?
If the former, it seems pretty clear that this does occur, if that latter, then I think the burden of proof is on you to show that it does happen rather than on Silver to show that it doesn't.
Just to be clear, which of these, or some other, is Silver referring to, and which are you referring to?
It's all pretty irrelevant when you consider that the last thing the Commissioner (Silver) would say is that the games are somehow "fixed" and the outcomes are "predetermined" by parameters other than what you see on the court (i.e., tanking to get better lottery picks).
Doesn't matter what he thinks/believes or what the evidence suggests, it's part of his responsibility as a Commissioner to protect the produt and not to admit guilt.