I haven’t read the book (nor do I intend to; I don’t really think Zionism is the issue for today).
That said, some quick thoughts about your question:
1. If the people to whom he refers had taken public stances on the issue, there doesn’t seem to be a problem of Lashon Hara unless his intent is specifically to publicize the fact that they had done something [he considers] wrong.
2. The Chofetz Chaim specifically notes that it is permitted to say, “Don’t follow the example of Ploni, who talks in Shul. He is doing the wrong thing.” Again, this heter (IIRC) is predicated on the question of intent. If, in saying so, your purpose is to use the example of Ploni as an instructional tool for your audience, you may say this. If your objective is to call attention to Ploni’s wrongdoing, you may not.