I believe the intention was that the free demo would be available only as long as it was still in development, then replaced with the paid full release. I could be wrong, though.
That may be possible. But I don't believe the full game is on itch.io right now. And since they aren't really overlapping stories (well there's some overlap) I kind of think both stand on their own. Of course, it's just my opinion, and the developer is free to do whatever they like.
