In Paul Fleischman's Seedfolks, why does Amir tell the story of the Polish woman who had been in a concentration camp?

Amir tells the story of a Polish woman he met, who did time in a concentration camp, in order to illustrate the point that you should not judge a book by its cover and/or reinforce stereotypes. You actually need to get to know an individual if you want to learn about them, instead of making judgments about them based on where they're from or the color of their skin. Prior to telling the story...

Amir tells the story of a Polish woman he met, who did time in a concentration camp, in order to illustrate the point that you should not judge a book by its cover and/or reinforce stereotypes. You actually need to get to know an individual if you want to learn about them, instead of making judgments about them based on where they're from or the color of their skin.
 
Prior to telling the story about the conversation he had with the woman, Amir mentions that Cleveland is a city made up of many Polish immigrants. He says that he was always told that Polish men were steelworkers and Polish women cooked a lot of cabbage. Despite what he heard, though, he mentions that he had never actually met a Pole until he spoke to the woman in the garden. 
 
One day, when the two of them are tending to their seeds, Amir notices that the woman isn't thinning out her row of carrot seedlings despite the fact that they are ready. He asks her why she hasn't done so and she says that the task "reminded her too closely of the concentration camp, where the prisoners were inspected each morning and divided into two lines" (Fleischman).
 
She then explains that her father was an orchestra violinist who spoke out against the Germans which, in turn, got her entire family arrested and imprisoned. This revelation causes Amir to view the woman as an individual human being as opposed to just a Polish woman. 
 
About the interaction, Amir says, "When I heard her words, I realized how useless was all that I'd heard about Poles, how much richness it hid, like the worthless shell around an almond" (Fleischman).
 
He then notes that he still doesn't know if she cooks cabbage and doesn't care to find out.