Sorry, AR, but that's wrong.
It's specific sins, not any mortal sin, and they aren't excommunicated only in the eyes of God. They are excommunicated in the eyes of the Church, just as much as someone who is excommunicated by a formal decree (called ferendae sententiae).
It's true that the church can't enforce the excommunication if she doesn't know about the sin, but that's the only difference. In Hitler's case that was obviously not an issue.
The whole "Hitler wasn't excommunicated" silliness is honestly a very moot point. Excommunicating Hitler wouldn't have done anything -- he didn't attempt to receive communion at any time after his teens -- and there was never much doubt about what Rome thought of him ... none at all after Mit Brennender Sorge.
"Dr. Martin Luther would very probably sit in my place in the defendants' dock today, if this book had been taken into consideration by the Prosecution. In the book 'The Jews and Their Lies,' Dr. Martin Luther writes that the Jews are a serpent's brood and one should burn down their synagogues and destroy them..."
Indeed, no historian has yet to put Martin Luther on trial for his incitement of crimes against humanity.