Realm First is a pretty big deal and seems very easy to detect if a guild is selling that title... so if Blizzard wanted to make that a bannable offense, they could easily do so, and should imo. ie: Store everyone's name who joined the guild within X days/hours of a world first kill, if there are a lot of them, then question the guild and find out what's up. Simple.
This is exactly the comment I was looking for. It would make the most sense if WoW's system would take a snapshot of all the players in that guild at the time of the 1st kill and allow them to gain that title if they too kill Ghuun. Folks are acting as if the twitter is clear, but the context of the question was that they were asking if the BENCHED folks will have their chance at their title.