I once had a manager who had a bunch of people leaving. They were not leaving him, the manager; they were leaving the situation the division was in that put the engineers in difficult working conditions. But the manager had HR interview several of his people, to see if he, the manager, was the problem. He said, "I had to know." (Of course, if he's that honest and that willing to look, you have a pretty good idea that he's not the problem, even before HR comes back with the results.)
I'm not here to dispute the figures that were shared in the article, I'm suggesting that the interpretation of those figures was off, as it overlooks the aspects of management that are out of the control of the line/middle manager.
Furthermore, I'm not disputing your own reasons for leaving, as I'm not suggesting the competency of line managers can't be the reason people leave their job.