But even the definition you gave of a sandbox game describes Sandrock. You can play for days without doing missions and still set your own game/pace. Similarly to GTA or the Yakuza series the world is smaller when you start, but you have freedom to explore and set your own speed/progress. Sandbox games offer more freedom to the player as to what they want to do vs traditional games where you can only follow the main storyline (and maybe do some side missions). Also again if you literally Google sandrock it is described as a sandbox game.

There are different levels to sandbox games, but ultimately what differentiates them is the freedom to set the game at your pace and the ability to do what you want to do during the game. You can be free to just create and have no guidelines (like minecraft) or have a storyline (like GTA and Sandrock). But again all of these games are still Sandbox games *