I disagree with the article. Perhaps I'm too strict of a grader, but a developer who meets the qualitative criterion here--essentially, a developer you don't have to manage much--is not yet a senior developer. She is on her way, but to me the distinguishing factor is the extent to which the developer can mentor junior developers.
If a developer meets the criterion of TFA, then she helps her manager grow the team linearly, as the manager can then focus more on others. But if a developer can effectively mentor junior developers, then the team can scale. This is not to say that a senior developer must be a "manager", but a senior developer not only understands the importance of craft and can practice it, she helps apprentices appreciate the craft as well.
If a developer meets the criterion of TFA, then she helps her manager grow the team linearly, as the manager can then focus more on others. But if a developer can effectively mentor junior developers, then the team can scale. This is not to say that a senior developer must be a "manager", but a senior developer not only understands the importance of craft and can practice it, she helps apprentices appreciate the craft as well.