I think that the statement is quite valid and represents the most important elements of the book. Amir introduces this to the reader when he tells us that Baba Khan ended his phone call with the words, "to be good again." This mere inclusion helps us understand that Amir is plagued with a need for atonement, something that comes out later on in the chapter. The shadow of "unatoned sins" is something that bothers Amir. He understands it and the power of his narration helps us understand that he himself is aware that some level of redemption is needed. The call to go back to Afghanistan helps to bring focus to Amir, and to us, that this journey has to be physical. The language that Amir uses to describe how he behaved on that fateful day in terms of "crouching" and "the crumbling mud wall" helps to bring to light the idea that he has done something in his past that reflected a retreat from embracing the better angels of his nature and displays an action that requires a level of atonement and reclamation. It is through the very first chapter that the entirety of the text helps to bring out the full force of the statement. What Amir undergoes and what he does is in his hope to represent the essence of the statement.