Weather extremes. Blizzards, deserts, hurricanes, life rafts... If you're stranded on a cool summer afternoon, obviously you can be outside for more than 3 hours, but without shade you might get a sunburn.
It's meant as a survivalist priority list. Build a shelter, find water, make fire if you can. Food can wait until tomorrow or the next day.
Still sounds implausible and far fetched. Why would you be outside without adequate clothing? (Also below zero can be fine in jeans and sweatshirt if the sun is shining and there isn't much wind). And in that case it would be more a heat issue and less like a shelter issue.
The other case also sounds more like a heat/water issue and less like a shelter issue.
I've seen that before and the 3 hours without shelter always cracks me up. Someone really needs to come up with a suitable replacement for the 3 hours part.
Heat index above 40? That's ridiculously low. 80 degrees F and 40% humidity is an 80 index and that's perfectly comfortable weather. Heck, I used to work manual labor outside for 8 hours per day in a heat index of 136+. It wasn't fun, and I was hot as fuck, but I did it every day.
The confusion is because you said "a heat index above 40", so I thought you meant the actual index, not the index of 40C with whatever humidity. 80f is 26.66c (sorry, idk if you use decimals in Celsius or not). The heat index of 136 that I used to work in would be 43.33c and 40% humidity, so about what your index would be. I guess it could be fatal if you didn't stay hydrated, but probably not in 3 hours. At least not for someone who's used to it. I did get heat exhaustion a few times when the temperatures rose to 46.11 C, thankfully never heat stroke. I hope it doesn't seem like I'm trying to argue with you. I'm just trying to have a friendly conversation. Sometimes it's hard to tell how things read on the other end.