I like Ms. Floyd's statement that, "A fallacy is an error in part of the reasoning that renders the conclusion invalid." Metaphorically, it is the one piece in the Jenga tower that, when removed, causes the entire structure to collapse.
Having gone through 14 of the lessons in this book I can see where 40 fallacies in one semester would be rigorous in the extreme for middle-schoolers. As you noted, it would give them little "time to practice and contemplate the concepts and skills". Twenty-eight fallacies on the other hand obviously dials it back a bit to an appropriate number for a semester. Plus the book is entertaining enough for students through the artwork and mock conversations to make it enjoyable. It seems like twenty-eight, is manageable considering the discussion and homework needed.