Yeah, but everyone sees the intro. Some people, on the other hand (many of them people who actually wouldn't have watched the intro in the first place) just want to show that they have knowledge that should only come from having beaten the game, whether or not they even understand it. I would wager that most people who say "Rosebud is his dog" or "Snape kisses Dumbledore" have no understanding of, nor desire to understand, the real significance of those plot points in the story; they're just showing us that they know the ending.
Maybe.