Sure. McCarthy was right that there were Soviet agents in America, and that there was a Soviet threat. But he was not the lone canary in the coal mine, blowing

Sure. McCarthy was right that there were Soviet agents in America, and that there was a Soviet threat. But he was not the lone canary in the coal mine, blowing the whistle on Soviet deceptions. He is not known for being the voice of reason when few would believe the reality of the situation. He is known for leading/inspiring witch hunts for Soviet spies, and ruining people's lives based on coincidence and thought-crime. When you say he was "not entirely wrong" without qualification, it is these things that people feel are implicitly referred to by the statement.