Seymour Hersh, in the last decade, claimed that Bin Laden didn't do 9/11, that Assad didn't use chemical weapons on his own citizens, and that Seth Rich leaked DNC info to Wikileaks. He's gone off the rails and there's no reason to believe anything he says lately.

 Moreover, each time he didn't merely claim he was convinced, he asserted that he had secret sources with great credibility who had access to all the secrets and were telling him this.