I think to an extent the mental impact of it is a necessary evil. The future resilience manifests as a drive to not find yourself in the same (or an equally difficult) position again — because it’s so emotionally devastating — so you fight harder to not allow it to happen again. This makes a person more driven in general.
Another aspect is that you’ve seen how ‘deep’ an emotion can be (traumatic) and so more ‘everyday’ emotional events can seem much more trivial, making them easier to deal with. Although, it can sometimes leave the person seeming ‘cold’ emotionally. One thing I found was I was less tolerant of people without the level of resilience I had, which I had to work on.
Of course, there will be some people that can’t endure the initial hardship and don’t develop that resilience. My impression is that most people do endure and find a way to come out of the other side, like a basic survival instinct, although that’s purely anecdotal.
Hell I guess you can describe them as "hard men" but I wouldn't want to be that way and it doesn't seem to make you more successful in modern society.
Not sure what else you’re expecting? I’m not advocating imposed hardship, just trying to give some context for why it can often lead to a more robust and driven person. It’s clearly not universally true.
I imagine there are lots of veterans that are able to cope and have become more robust. But there will always be mental health aftershocks, because that’s why it was a hardship in the first place.
I think the phrasing can come across as a bit macho, which I don't think is the point. It's about resilience.