How do you figure? According to The Writer's Tale, RTD knew about Moffat taking over well in advanced of when it was publicly announced, and the book contains an e-mail exchange between RTD and and DWM writer talking about Moffat taking over as showrunner dated six months before it was revealed to the public. Why wouldn't RTD and Chibnall have been discussing the matter before it was public?2) The search for a new showrunner is not something that would be in Chibnall's purview, and RTD would not have been able to share anything about whatever talks he may or may not have been having vis a vis Doctor Who with anyone, so, no, Chibnall would not have known about RTD returning until the General public did.
I mean hell, Tennant knew Moffat was taking over a year before it was made public, the two of them even had conversations about it on set while filming the Library story.