He basically announced that he knows a secret worth millions. Criminals and state actors might do everything they can to get this secret, starting with trying to hack him, over bribing him, blackmailing him, serving him secret court orders, or even physically assaulting him with the famous wrench.
Even if you are a seasoned security researcher, saying "I know how to get into any Windows PC by sending someone to a website" paints a huge target on you.
Although the danger is kind of abstract, there is no security gain from tweeting about this, so I'd say he shouldn't have done it.