Well, 30% that's not from the individual managers still leaves plenty of people who have never seen this even once in their careers. So there's plenty of, if not exactly counter evidence, then... counter experience?
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 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.)