I've always understood "middle manager" to mean: manager of a manager, but subordinate to another manager. This article uses the term to mean: entry-level manager.
What's the common understanding of the term?
I think this article is describing Frontline managers, but agree with your definition of middle managers
I’ve seen it used both ways. Many use it for the lowest level of management because they are middle men, in between the real management and the real workers.