Jim never gave a concrete number but I mean it makes sense that a majority would assume he was talking about the third highest grossing film today and not ten years ago. Also I won't blame a forum member for believing something that was reported in all the major trades. I won't hold Cameron blameless as well, He knows how the media is and how it would interpret his words.
Anyway ten years ago Avengers was the third highest grossing film at 1.5 Billion so that number matches with what others here have computed.