I agree with you.
It is not clear how Eq. (12.4) was derived.
The book simply states:.... using feedback techniques as described in chapter 8.....
So the reader is invited to read chapter 8 - hoping that he will find the corresponding part.
More than that, this sentence sounds a bit confusing because Eq. (12.2) is also, of course, the result of feedback.
My recommendation: Do not spend too much time for clarification.
It is not very important.