I have no problem believing the story, but I wouldn't necessarily put it beyond the placebo effect. While panic attacks can have very real physiological symptoms, their origin is almost exclusively psychological. So if you truly believe that something prevents panic attacks, there's a good chance it will actually do so - no matter how realistic it is from a purely objective, physiological point if view.
Have you ever tried it? I've found it to be really effective for anxiety and other things, though I'm not a hardcore practitioner.
Even though it sounds like hippie bullshit, he accidentally stumbled onto a huge body hack to control his autonomous nervous system. It has also been validated in studies. It's also impossible to ignore all the extreme cold things he's done without getting frostbite.
You write "It has also been validated in studies" while your supporting link says "but Hof’s extravagant claims of health benefits are not supported by scientific evidence" and "Conclusion: The science is lacking".
I missed that part of the article, but I'm not sure why they drew that conclusion. Based on the findings of the studies it cites further below, they don't really put forth an argument as to what's wrong with the findings, I think at best one could say based on those specific studies that there is maybe weak evidence that deserves further study, and I will admit the sample sizes are small. However 2 out of the 3 studies that she cites actually do back up Wim Hof, but just not in an extremely conclusive way.
To me, it seems like the author cherry picked those studies to support their conclusion, or it was written before additional studies came out that validate the physiology behind his method. Based on the date I'm going to say cherry picking. There are a few more listed on his own website, I was going to use that as a citation but was trying to use a neutral source.
One thing I noticed was it was meant to show that people who followed Hof's training had several measurable differences to those who did not.
What it doesn't do resolve if all three parts of Hof's training ("meditation, exposure to cold, and breathing techniques") were essential. The article in The Guardian seems to suggest that these aren't all needed, albeit only because two of them weren't mentioned. [1]
I also noticed that the nine people who did the training also went to Poland, while the control group did not. That means the study cannot exclude the possibility that air travel, the food they ate, or other shared factors had an effect on the result.
[1] I came across https://www.sciencedirect.com/science/article/pii/S096522992... - "A systematic review and meta-analysis of the effect of whole body cryotherapy on mental health problems" - which suggests the benefits primarily come from the temperature.
It ends as you suggested: "Results indicate preliminary evidence for WBC as efficacious add-on intervention for mental health problems, especially depressive symptoms. Further research in the form of RCTs with larger numbers of participants is needed." :)
I would be very interested in them if it worked, but it sounds so hokey.