By an interview I watched, this game has only been in full production after the release of shadowfall, so they had about a year and a half to build all assets and content shown on the demo, which according to guerrilla, is pre-alpha still. Preproduction started after KZ3, with just half dozen team members using their ps3 engine and KZ assets, for the first tests and prototypes. That was halted when the release of Shadow Fall approached so the whole team could concentrate on finishing it up. Only then the entire team started work on Horizon, with an already released game on ps4 under their belt, on a pretty feature-full proprietary engine, so this game is being built by people more experienced than most being showcased so far, great! For an open-world game, in development for less than 2 years, I think they have a lot done.