As a vet myself, though without PTSD, I can attest to this. I have been a semi-serious alpine climber and the reason (for me) that this works is because you have to be focused on the exact moment and precisely what you are doing at every instant. Crossing the Grand Couloir on Mont Blanc a few years ago, there was a serious rockfall problem (think Volkswagen-sizes boulders flying down the mountain,) the process of preparing for such a dangerous (and foolhardy) maneuver, puts your mind in a good place ironically. You don’t give a shit about drama when you are purely focused on your next step. Crossing a cravasse-filled glacier roped to your partner has a similar effect. Combining that with the slight hypoxia at altitude and it’s an amazing curative. Not even to mention the physical suffering of moving for 10+ hours, loaded with gear.
I get a similar benefit from flying small airplanes — the intense focus on every detail is like a forced meditation: you CAN’T be depressed because there’s no room in your thought for it.
The feeling for me continues several hours after flying or climbing — you feel almost weightless and you are very deliberate in your actions (driving after flying finds me less impatient, much more spacially aware and just calmer.)
I am only speaking for me, but I feel there is something too this — not just for depression, but cyclothymia and mania as well.
High altitude meditation!