In the military I was in a variety of life threatening situations (both forward deployed and in garrison), and I was quite happy. The most interesting realization came when in water training, where there were times I honestly felt like I was in mortal danger. I was ok with it and still enjoyed life. Eh, maybe I'm a weird one though, I always found humor in it.

I'm not saying people can't find happiness in those situations (in fact civilians in those situations are often amazingly upbeat all things considered).

I just mean the bit about external factors almost implies external factors barely affect happiness is silly, external factors are also a large part of happiness.

I don't think they are saying external factors don't/can't affect happiness. I think they are saying that manipulating external factors is an inferior strategy for improving happiness.

So some people have to play the game on hard-mode. No reason to just give up. Find a way to make it happen and try to have some fun in the process.