it's still a quite a stretch.
Have you ever run a homebrew PnP roleplaying campaign? If you did, this wouldn't seem like a stretch.
Let's take D&D as it's the most known example. If you're going to create your own setting, you'd first look at what's been done before, for inspiration. You'll also know that one of your main goals, before you can start playing, is to explain where characters come from. You need to account for races and classes. You need a history for each race, an environment where they live or many, culture(s), political relations etc. You also need cultures that lead to characters having the available classes, places where skills are taught, groups of experts, their role in society, etc.
But races and classes are archetypes, or even stereotypes. So you start thinking in those terms. Relations between races and between nations are informed by those archetypes. Hell, even Gods are just that: idealized archetypes, and each has a magical portfolio tied to his archetype. It's all built like that.
As a designer, those are the atoms you work with. And everyone that's in that field, at the time anyways, has passed by D&D. It's the basis, the standard. If you've learned D&D archetypes, worked with them as a young kid, they stay with you. You still go back to them when designing, they're part of your design tools. Notice how PB's factions are just that: archetypes. This is how they design.
So the high elf archetype? They know it for sure. Sure, in Germany, D&D's not the dominant RPG, it's Das Schwarze Auge (The Dark Eye), but it's no different. It has that same archetype, except they're called Firn Elves. I haven't played DSA, but wiki describes those elves that way: "live in the north, in perpetual ice; for them, life is a hard struggle." Here we go with the
cold again. But frankly, their knowledge of such an archetype need not come from D&D or DSA, since those are inspired by folklore, faiiry tales and such. It's all there.
It seems obvious to me that PB's designers had first their idea of Elex as something that numbs emotions and makes you cold. And then they linked it to a "cold" racial archetype to embody it. And a "cold" environment to house them, etc.