Can they decide to leave that hub and all those paths behind to say go to Chult to dig in ruins for treasures of a lost civilization? If yes, then it sounds like a sandbox. If no, then it's not a sandbox. It's a box. A large box if it has that much in it, but a box none the less.
Just as an aside, I never really understood why sandbox games that are all about the freedom to go anywhere and do anything are called sandbox. Sandboxes are limited in size. You can't leave that square area of sand and still be in it.