The demo has a lot more content than you can cover in three hours they let you play. The main story is adapted almost 1:1 from Champions of the Continent but there's small edits here and there like a character moved from one place to another or a flashback shown at a different time etc. and it's all tied together with a new story about your MC which is basic stuff but I guess it works? I'm honestly just happy CotC storylines are gonna be shown to more people because they're pretty good. I think the game expects you to explore and recruit more people before diving into the storyline however because the enemies are tuned a bit higher than I'd expect if you just rush forward. There's some new music already, it's pretty good. The English voice acting is also a pleasant surprise. I wish they had updated some of the OT1 assets, the game looks very dated even for its artstyle but at least they updated some of the old CotC cutscenes to look less flat.
Overall I think if you wanted to play the gacha for the story but were turned off by mobile game nature of it all you'll enjoy this one if you hated Octopath to begin with this one won't turn you around.