Scout is not aware of the importance of Atticus' advice until she stands on the Radley's porch at the very end of the novel. She doesn't respond to Walter Cunningham because she is seeing things from his perspective. She is utterly unaware of what she's doing, other than being polite to grown-ups, as her father has taught her. Similarly, she shows curiosity about Calpurnia's church, and First Purchase's style of worship, but she doesn't understand the difference between her church and Calpurnia's. She can't comprehend Miss Gates's comments about "those people." And she doesn't understand why Jem and Dill are so upset at the outcome of the trial.
The power of "To Kill a Mockingbird" is that the narrator reconstructs events from her childhood, from the perspective of an adult. At the time they are happening, Scout has no clue of their significance; to her, it is only the story of how Jem's arm was broken. Jem has the maturity to interpret what he sees. All Scout does is record.
So the short answer is that Atticus's statement is of no use to her at the time. It is only in retrospect that she can put together the pieces of the story and grasp how she has, unconsciously, been affected by Atticus's advice.