Looking at annotation as a genre
The preface didn't lie. This book looks at annotation as its own genre, not as a mere addition to the act of reading.
However, I feel that this definition of annotation is too broad, despite the authors' elaboration. It gathers marginalia, commentaries, and peer reviews altogether under the annotation umbrella. According to it, if you've ever posted a comment on reddit, you've annotated the original post; similarly, if you've rated a purchase on Amazon, you've annotated the product listing; and so on.
Regardless, if one accepts this interpretation of annotation – that is, augmenting text that already exists – then the rest of the book makes sense. Not that anyone ever contested the utility of noting thoughts in book margins and sharing them in online communities, peer reviewing scientific papers or policy proposals, or even leveraging viral trends for social and political messages. In other words, what is written about annotation will not surprise anyone but will likely elicit the question, "Is everything just a note then?"