@Deicide:
Rocks D. Xebec was said to have broken several taboos and to attack the WG directly. The Rocks Pirates probably came to learn a lot of secrets from Celestial Dragon documents and government facilities. It doesn't meant they got the whole picture, but they probably learned about several things: Poneglyphs, Joyboy, the Ancient Weapons… None of those are common knowledge after all.
This chapter made me question one thing. If Kaido knows about someone "becoming Joyboy", then he has no way of becoming Pirate King without a Joyboy, right? So, maybe he tried to have a Joyboy of his own. Maybe that's why he begot Yamato? Maybe Yamato's mother was a D, thus making Kaido's son also a D, and that's why Kaido wants him to be shogun, despite his rebellious nature.
-
As you say, they could have learnt about Joyboy from stealing these secrets to WG and CD direclty. Although they could have also known through someone in their crew who was able to read poneglyphs.
-
'Pirate king' is just the term the WG gave Roger, it has nothing to do with Joyboy. The prophecy carved in the poneglyphs is unrelated to becoming pirate king.
Kaido cannot become Joyboy but can become Pirate king, no need for anyone in his crew to be Joyboy.
–- Update From New Post Merge ---
@andre:
Kaido hasn't been to Laugh Tale because he doesn't have all of the Road Poneglyphs and doesn't know that Laugh Tale is (my belief in the location of One Piece is in the spoiler tags)
! at reverse mountain
I would assume the other person who was trying to be Joy Boy Kaido is mentioning is Roger, but I could be wrong.
Exactly, Kaido has not been to Laughtale, but who knows if Rocks has.
Roger learnt about Joyboy once he got to Laughtale, but never wanted to become him, this was not his goal. He accepted the Joyboy would only come around 20 years later. It is only after learning about the Joyboy prophecy that someone could want to become him.
My guess is that Rocks heard about it, became obsessed about becoming Joyboy (he had all the requirements except for the timeframe) and this is why Kaido knows about someone becoming Joyboy. He may not know the whole prophecy though.