Up until that moment when Kan Pi said that Shin was too positive because everything Shin had said was that sure, a bunch of people do many bad things, but most of the time there will be someone that thinks of them as good because of one reason or another, implying that we all have the best of intentions (we are good) even though we sometimes do evil things. And then Shin continued to talk about how everyone inherits the will to live and the strength of our friends and even our enemies, implying that even in war we become better and stronger.
Shin was absolutely implying that people are good and naturally improve even in conflict. That argument can be directly related to the idea that we are born good. Kan Pi's conclusion was explained right there and there, I'm just saying the same thing he said but with more words.
And the reason why everyone, included that lady, were so shocked by Shin's statement that Kan Pi actually believed in the good of people, was because up to that point and way before this particular debate with Shin began, Kan Pi had advocated and preached that humans are born evil. It's like saying that a proven racist person in truth loves other races, or that a person that has always shown support for trans is actually sexist and homophobic.
And all this explanation is based on just what the past 4 to 5 chapters have shown, I'm not even referencing the historical context of who was Kan Pi.