what has jem learned that would lead him to develop the idea expressed in the final paragraph of chapter 23

Even after all the events of the trial, Scout continues to believe that all people are the same. She believes all people are "folks," and that they are neither all good nor all bad, and sometimes they act out of weakness. She can't determine what makes her family "better" than the Cunninghams. Jem seems to still want a reason to explain why some people act the way they do; he feels that he has outgrown Scout's viewpoint and needs a new one that is calibrated to his more mature mind. His comment about Boo shows that on the whole, he is feeling mistrustful toward humanity.