Some "alternate history" is not sf; it can just be historical fiction.
If you think about it, it's interesting why "Guns of the South" is classified as (and feels like) alternate history and not time travel, even though, schematically, it involves a group of people who travel from the future to the past and use their advanced technology and knowledge to try and change the past. Conceptually, it seems very similar to "Lest Darkness Fall" (from the 30's) and the numerous time-travel stories that followed.
But I think what makes it different is the focus. "Lest Darkness Fall" (and the H.Beam Piper stories, and "Doomsday," and most traditional TT works tend to focus on the time-traveler him or herself. In "Guns of the South," the perspective is from the past and the protagonists are the confederates of that period, with the time-travelers being very minor characters. That distinction isn't always easy to draw (I'm not sure where I would put 1632, really), but I think it is meaningful for GotS.