Please, do you really think they've only been working on Platinum this whole time?
When they were making Platinum, they were also making HG/SS, and theres easily much more to the game than they're letting on, I wouldnt be surprised if they have it finished already.
There was an interview with Gamefreak somewhere about this very thing,they've been doing these games for much longer than everyone thinks, and they've further with it than everyone thinks, and they still have some surprises left.
This Pokemon Sunday update stuff is to drip-feed everyone, to make them watch Pokemon Sunday, and to make them more excited about the games. I bet Pokemon Sunday has all the information already, they're just doing what normal media do, and drip-feeding the audience.