If they use a random guy claiming to be Joakim Mogren during that interview then I have a reason to believe that The Phantom Pain and Ground Zeroes are 2 different games because I see no reason for Kojima to keep it a secret if it was the same game. Kojima may not be ready to fully reveal the identity of The Phantom Pain because if it really turned out to be MGS5 then this will overshadow Ground Zeroes big time and you will end up with 1 game going head to head with the other specially if 1 is on PS3 and the other on PS4. Just my 2 cents.
Then why the fuck would he announce The Phantom Pain if it was going to overshadow MGS:GZ?