It might have been part of the core story all along, but not necessarily as an explorable location, which, IIRC, was what the stretch goal was about. It could have been planned as part of the interludes, for instance, or in scenes where key information would be given even if there was no player control of a playable character... if this makes any sense!...
I'm only going to quibble with this because of the phrasing in the original Kickstarter update. Recent bits that I've seen indicate that it likely plays a bigger role than I was anticipating. However, in the Kickstarter post on the subject, they mentioned that they'd been "toying" with the idea of including the HoAW in DFC, and then went on about how important it was in ways obvious and less obvious to various characters. And that if they hit the stretch goal, then obviously they'd be able to do just that. Which makes it sound like it was something they wanted to do, they had some great ideas for it, but they could do without if needed.
But again, storytelling isn't a static thing. Plots, plans, and schemes grow and adapt and change in development. And I'm totally cool with it if the HoAW plays a way bigger role than I pictured when that stretch goal was first mentioned.