@Mew Some people tend to be more resilient than others, like I said in my comment, mental health is a factor when people go through stressful events, in the next chapter Baek admits himself that he was just hyperventilating, but still how he reacts it's not to crazy or cringe tbh, specially when we see how he has experienced those same symptoms since he was a teenager, that's why he doesn't practice the sports anymore.
There's infinite ways to react to something that scares us and honestly he reacts more normal than some people I know who has phobias for animals and other things even if they haven't experienced anything bad lol