The point is Slam Dunk might want to play their cards early to sell more tickets before the Download announcement because if Download announce Green Day, Jimmy Eat World, Yellowcard, Frank Turner, Beartooth and someone like Pierce The Veil or Avril Lavigne someone may choose to go Download over Slam Dunk.
The story behind these posters is this; Over on the discord, they have a stage predictions thread, much like the one on here. One of the users creates a poster from his guesses, just a bit of fun and to be fair, it makes it more interesting than just a list of names. At no point does he say they are a leaked line-up.
These posters often make their way into the wider world via Reddit or one of the Facebook groups, with someone then making out it's a leak. So what has started out as a bit of fun, ends up being something people either believe, or get angry about.