In other words, it wasn't just a minor adjustment that could easily be gone back on. It was a radical paradigm shift. In order to argue that Bonhoeffer later repented of his pacifism, we need to find a similar radical shift. The authors will argue we don't find that shift.
While I agree with them, I don't buy their argument that Bonhoeffer wasn't really actively involved in the resistance and the plot to kill Hitler. More on that later, when I write the review of the book as a whole.