Earlier quoted context omitted.
My try a minute ago made it seem that it will stage every line in the region you make. Subline regions don't come up that often. Or did I try something wrong?
Yeah, it works at the line level (as indicated by the way it highlights the selected lines) which I think is a limitation as much of git itself as magit. (I'm not sure what subline staging would even do.) But the author isn't trying to stage a region, let alone a subline one. He's trying to stage a whole hunk and says its only working if point is at the start of the line.
Subline staging makes sense for some changes. In that, if it does subline indications of what changed, I would expect I could accept one of the changes, but not the others. That said, I agree it is likely a limitation of git, as much as of magit. And, again, I can't imagine this comes up that often.