I am not visiting the link and my comments are not intended to address anything found in the linked article.
Training can help.. experience and repeated exposure to certain stresses, danger and the pressure associated with such situations can help desensitize. People are people but we are all wired a little different. Some people simply cannot manage high stress and are often a liability to themselves and others. Some people can learn to deal with high stress over time and still others may become desensitized to a certain degree via repeated exposure. This not always one or the other but rather a combination.
Many responses are simply autonomic, innate, instinctive and hardwired into our brain. Some of these responses are good and some can be hindersome. Understanding what to expect can go a long way to curbing or mitigating unwanted responses. I have never really benefited from breathing techniques and simply had to think my way through it, aka a mental roadmap. Certainly if you can escape or avoid danger, you should probably do that. If you cant, I would be inclined to focus on what needed to be done and set the rest aside in a separate mental box.
A person should probably not forget that dealing with and managing stress can extend beyond the specific crisis.