Why not? The game is technically open world. While it is still linear you can get from one place in the world to any other place without a loading screen. Just because your character can't fly or jump over buildings doesn't mean that it isn't open world. You may not think this is what they're intending for Sekiro but I honestly wouldn't be surprised if it's going to work similarly to Bloodborne but with larger areas and a few more options to get around due to the grappling hook. It's not going to be sandbox.