Just as it's stated here:
"Apparently the demo that was seen at E3 2015 was already completed on the PS3 by July 2010, but once they shifted their focus to PS4, they ported the demo to the new hardware and showed it at E3 2015."
Ueda and co shifted their focus to PS4 in 2012, so that's when porting the game over to PS4.
If they started working on the PS4 version of the game in 2012, then they began developing the engine then, would probably require the engine before assets and their code could be moved over or tweaked to run on PS4.
Gen Design and SCEJ have had 3 years, nearly 4 years come 2016, considering how well the game's already and the fact that it's already been seen by journalists behind closed doors, played through different section and in slightly different ways it's not going to be like before and I highly doubt this will be delayed into 2017, otherwise Yoshida and the other bosses at Sony probably wouldn't have bothered showing it.
As for the graphics side of thing, it's clearly a huge improvement over the PS3 version, just from an visual point of view, the fact it's already running natively, with nothing sped up on the frame rate front and no extra tech specced up to make it look right means we're seeing the true PS4 version of the game.
Comparing Gamersyde footage to YouTube the difference is huge, the extra details are easy to spot, the events we're seeing aren't scripted as they were in 2010/2011, it's all dynamically calculated physics, tonnes of particle effects, much higher res textures, more geometry, those feathers on Trico react to the wind and don't just move based on some predetermined code.
We'll see what happens, but the fact Ueda is even talking to the media should be a good sign or that they're able to release such information.
If we don't see any new stuff or hear any new details before the end of this year maybe have a little concern then, but I don't think that's likely to happen.
Most likely we'll get a release date by the end of the year, same goes for Horizon: Zero Dawn, probably Dreams and if Morpheus gets a release date and other details then RIGS will probably get one too.
IMO all of the above will be released by June next year, along with Street Fighter 5, Uncharted 4 and Ratchet & Clank.
