He probably didnt. Or he wouldnt had said to Hulk the first time he faced him " Thor would have been a better match to me, than you " or something like that.
Not to mention, in the trophy room, when he was talking with Rick Jones, we learn that alot of heroes died or dissapeared (?) during the nuclear wars.
That absolute idea that Maestro killed every hero on earth doesnt come anywhere the comic Peter David wrote, last i read it.