Mental toughness isn't anything covered in Deep Survival - what they talk about is the predisposition of people to react in certain ways in survival situations. The thing that makes the most difference between survival and death in the sorts of situations in the book is not toughness, but the ability to accept the situation for what it is and not panic and start to make the poor decisions that lead to being more lost, more hypothermic, more dehydrated and more at risk.

As a mental health practitioner and SAR volunteer I did find Deep Survival interesting, but the psychology of survival is more about how lost people behave - and one is not always lost when in survival situations. We have mental maps, a set of beliefs informed by myths, half truths and our own experiences, and a mindset that's determined by all of that. Survival skills training can offset some of it, so can research and gear testing, but in the end we sometimes make assumptions that lead to more mistakes and more assumptions, bad choices, and eventually we are left with open cases years later, like the hunter who vanished in a forest he knew well with only his guns, some bullets, and the clothes on his back. Massive efforts to locate any trace of him have failed despite knowing exactly where he left his truck full of camping gear and food.

Mental toughness is the sort of thing that leads to going backpacking again after learning from disastrous attempts, or continuing to rebuild oneself to physical fitness after devastating illness. It can play a part in survival but it's more the panic/not panic factor plus education that can help you out when you're lost. One of the things that Deep Survival won't tell you is that children have a higher survivability rate than adults - children who are lost in the wilderness stop to rest, drink when thirsty, and depending on age, are more likely to just stay put than adults are. So they are usually found unhurt and alive.