I don't think being a RPG or not is really where the difference is. I would say that something like Red Dead Redemption 2 and even GTA are not only open-world but are also sandbox, for example. Far Cry, Assassins Creed, LA Noire, or even The Witcher series are open world but I wouldn't call them sandbox myself. And most RPGs historically have not been sandbox either. And, you can also have a sandbox that doesn't have a 3D open-world or even an "open-world" at all. So, Outlaws could theoretically be a sandbox in this context, RPG or not, but I seriously doubt it will be. But I fully admit that the dividing line isn't clear cut and I don't have a perfect water-tight definition to offer.
Some people these days even call Assassins Creed an RPG while for others what we call an "Action RPG" isn't a "true" RPG. There's also a lot of historical contingency for what we call X or Z, so it's not always that helpful. Table top RPGs have also undergone huge changes in design during the last 20 years for what is to be expected from an RPG, but most people have missed those because they only play old and well established big IPs like D&D. And fanboysm and tribalism is rampant in that comunity too, which further muddles the discussion.
The only thing we can do is to be a bit more specific regarding what we mean ourselves when using a term while trying to take into account the term's historical usage. Other than that we just have to accept the unavoidable mudiness and imperfection inherent to most definitions.
Anyway, I think we all have a good idea of how Outlaws is gona play out, so I don't expect any real surprises there.