To be fair, do you really think Bernie Sanders would stand on the stage and call her corrupt? Its no difference than Obama saying that Hillary was "likeable enough," even though from his body language and the tone of that debate we all knew what his real answer was. This speaks nothing as to whether that charge of corruption is true or not, mind you. I don't really see it as a counter, but it will play well with audiences that won't look into the deeper nuances of elections.
But that's just the thing. This entire primary season Bernie has consistently implied that Hillary Clinton is corrupt without outright saying it. The implication is so obvious that not even his campaign denies it anymore, they just won't say the words "Hillary Clinton is corrupt." But they keep saying "The system is corrupt. A politician is corrupt if they accepting money from big corporations. Hillary Clinton accepts money from corporations, soooo..."
So it was a pretty major deal that, for the first time this entire primary, Bernie was directly asked to name ONE instance of Hillary displaying that she is bought, and he couldn't give one example. He just handed the Clinton campaign a whale of a counter.