Many walking simulators are very linear and railroad you into doing specific things and going to specific places. An example of this is Firewatch.

IMO the defining feature of a walking simulator is that they have no actual mechanics. Many games tell stories through exploration, finding audiologs etc., but they usually have other gameplay mechanics like shooting, puzzles, survival, whatever. Most gamers see gameplay as pretty essential to the medium.

A walking simulator strips all that away and leaves only the most basic possible mechanic: walking around. Most don't even let you interact with the world or with characters. I think this precludes this game from being a 'sandbox' as the article suggests.