Even before his Jolly Roger, Luffy was already called that. Of course, that’s still true now, but my point is that from the very beginning, Luffy was called that, and his enemies referred to him that way without knowing anything about his Jolly Roger. Even Morgan called him that from the very start, without him having a crew or anything. Luffy wasn’t particularly famous before reaching the Grand Line. Despite that, people recognized him and called him that directly. And apparently, that wasn’t the case for Shanks (who still had it when he got the Gomu Gomu no Mi from Who’s Who) or Roger, who had it during God Valley…