The devs wanted it to be unique, which is totally fine and understandable. Not every game needs to be open world.
This feels like the right choice of focus for an AC game, the ACs themselves, and well thought out map design.
It’s usually painfully obvious when you hit a game where the dev asked themselves (or had someone concerned about profits ask them), “Should this be an open world game?” Map pin collection sets in (every open world Ubisoft game), empty unused areas of the map with minimal detail that just feel like an area to grind juxtaposed against dungeons with fun and interesting puzzle mechanics and enemy placement that utilizes combat mechanics (Horizon).
It’s nice to hear a studio that’s aware of the type of game they’re trying to make, and the fans they’re making it for.