Un-Developing

I have been reading a book, "Elements of Positional Evaluation" by Dan Heisman. In the book he talks about development, and how it doesn't always mean simply moving your pieces towards the center. Then he shows an example position that arises out of a variation of the Ruy Lopez. He says that Nb8 is considered a good move for black in this situation. Why is this so?

Nb1 is good for two reasons. First, it allows the light bishop to attack through the light squares including e4, a target from the very begining of this variation in the Spanish. Second, it allows the re-positioning of the knight through d7 and finally into c5 attack again e4 while at the same time annoying the enemy bishop. The point is that the knight is no longer needed to defend on e5 so it can be used for the two reasons prior said.

sth is wrong with this... Maybe Black played 3.a6? instead of 3.Nf6!