Open World usually implies that you can just go
anywhere and access most of the content in whatever order you wish (even though the main plot and individual quests will obviously follow their internal order). If a game has a single interconnected world, but the progression is still sort of pre-determined and you are railroaded into new areas a few at a time and in a somewhat pre-determined order, I think it's better to just call it non-linear or a Metroidvania (even though I think Metroidvania implies a very specific way of progression, that consists of you backtracking to older areas but being able to access new routes from them thanks to the acquisition of new traversal abilities).
Just so we're clear, I'd call Dark Souls non-linear, not open world. Open world = no order of progression to reach different areas of the map. Non-linear = areas still follow an "order" and force you to "play" them before you can reach the next, even though you can pick multiple directions and they may interconnect in a complex way.
Skyrim's map:
Dark Souls' map:
There are also "hub-based" games where you have "levels" in practice, even though they are not presented as such, and you simply access the various areas of the world from a single central hub that connects all of them, but each one of them is self-contained and they don't connect with each other. (I think Strife from 1996 pretty much worked like that).
I didn't play this game and just glancing at the longplay it's hard to get a good idea of the world structure, but the hunch I'm getting is more non-linear than open world.