NOTE: I just want to point out that the majority of my heritage is not Indigenous, and even though these characters are fictional, it is not my place to speak on another's behalf for their feelings, as I have not been through their experiences. In saying that, all of my analysis here is based on what I have experienced or noticed in real life or throughout my academic career.
I think in this scenario, there are two types of people who "twitch in their seats" when they hear difficult stories:
1. They are ignorant and refuse to believe what they haven't experienced with their own eyes or;
2. They simply cannot fathom the reality of the story, because it is so difficult to grasp without having experienced it themselves.
For example, many people are ignorant when they refuse to believe that the Holocaust was real or they believe it was way over-exaggerated. (Those people dishonour the dead that lost their lives in that Genocide, but that's a whole different topic.)
The novel doesn't mention whether they are all there for the same reason as Saul, so when those younger people were uncomfortable, again, it could be for 1 or 2 different reasons:
1. Perhaps some of them are not Indigenous, cannot relate to Saul's issues, and become uncomfortable because they refuse to believe Saul, or;
2. Perhaps they have indigenous roots and cannot fathom that Saul went through this experience and they did not. (Or perhaps they are not Indigenous and again, cannot fathom that Saul and other Indigenous children had these experiences, and because of their own privilege, they did not have the same upbringings.)
(Many people in the world refuse to acknowledge that residential schools even existed and some think that they were exaggerated. I know from my own experience, my Godmother and I have gotten into fights because I would tell her that I had learned about these schools, and she would say that some of her family members lived in these schools "and had a GREAT time!" She would refuse to acknowledge that some people had worse experiences than others because it was beyond what her experienced relations to them were. We see this happen in real life, all the time.)
Overall, the younger people were probably made uncomfortable by Saul's story because simply put, it was something they couldn't relate to. This either made them get angry because they were so mad at the system for harming Saul the way they did, or it made them angry because they hold the ideology that "If I haven't heard/seen/imagined/experienced this before then there's no way it really happened. So he must be lying."