You know, my guess would be that they know that Tennant isn't going to be there, but that they're not even going to try to start casting until after the specials. It would, as you say, be a very long time to keep a secret. (That said, given Who history, it's possible that whoever they want to cast has already been on the show as a guest star, but they're unavailable for a normal season start, so they're holding the new season until they are available.)

They're also, I note, completely silent about the doctor's assistants for the specials. I wouldn't expect Catherine Tate to be back, but they also say nothing about Agyeman. I wonder if, after this season, they'll maybe shift her permanently to Torchwood (where she'd be a better fit, really), or if she's completely done after season 4.