I'm curious because it's only for next-gen, yet the game has been in development since 2009 when next-gen (Xbox One/PS4) hasn't came out. Was it originally intended for last-gen? Or they already had the next-gen specs and were able to work on it since 2009?

Way back in 2009 they were just doing the conceptual art and storyboards, they didn't need to worry about system specs back then. Skyrim was the game they were actively building after they finished with FO3's DLC.

It doesn't matter when they started it. Oblivion took years, was in development from 2002 and the 360 came out in 2005. That's how game development works.