Telegraph cycling podcast seem to be reporting that as fact recently too.
Is it the right move for him? He's going to be behind TvG but maybe he realises he's not going to successfully lead a team at the TdF for anybody. Might give him another tilt at the Giro though.
http://inrng.com/2015/07/team-sky-budget-finances/
Interesting reading if only for the fact it mentions Sky letting Porte go to BMC.