Don't expect any book to be 100% correct.
Every book that I have ever read and every book that I have ever written has contained some errors (and I don't mean just typing errors).
The AFE statement is the conventional wisdom on this subject.
The author of the PPL Confuser probably wrote his explanation to fit what was then the CAA answer to the exam question.
The question has since been updated. Neither the 30 degrees nor the 25 degrees nor the 50% speed change argument will get you exactly the CAA answer. But they will all get you an answer that is closer to the CAA answer than it is to any of the other options.