Jekyll would disagree.
According to most definitions of "CMS", it is. With static website generators, editing, versioning, publishing, etc is done externally. They aren't systems which encompass all of this. They just punch some content into your templates and write the result to disk. That's all they do.
From what I've seen of Drupal, the interface exposed to content creators is awful. It works (mostly), but it ain't pretty.