I'm always amused by how people on here post under threads and speculate about so much when they have absolutely no experience about a particular race/topic/sport. But such is the life of the LetsRun Messageboards! Granted I'm a N00b at skimo/skiing, but I've done a few ultra/mountain races in my day.
Any way consider some of these names:
Kilian Jornet
Francois D’Haene
Xavier Thevenard
Rory Bosio
Pau Capell
Nuria Picas
What do they have in common? Well they've all won UTMB (some multiple times and many in CR performances). They also ski quite a bit (mainly Skimo....or "Ski Mountaineering" or "AT ski" or "Randonee" or whatever you want to call it!).
Even for top American men (recent best performances at UTMB for American men) with a pair of solid 3rd place showings in Tim Tollefson (he does skimo in the winter a bit...esp since Mamouth gets dumped on!). Max King specifically told me a few years ago at a Golden Series race that skimo in the winter was the biggest factor that improved his climbing.
Sure, skimo is super niche and specific to "mountain running"/mountain courses. It's an expensive sport with a lot of specific gear. But consider the parallels to UTMB training/prep/demands:
1. You can rack thousands of feet of vert at high altitude up steep grades. Rea Kolbl just did 55,000' in 24 hours (for example). The action is much like. "power hiking" uphill, while downhill skiing requires good quad strength and coordination. That is the kind of specific mountain leg and core strength one needs to race well in Europe or even on a course like Hardrock. It's why pure roadies often get rocked in ultra-mountain racing initially. Their leg muscles aren't used to grades over 15-20%. Most American ultras and trail races aren't like that....but go race in Europe and see what I mean!
2. You get very coordinated with poles and using your arms. Using trekking poles at UTMB vs using them for uphill skiing is a very similar action. I'm using poles at UTMB.
3. You get used to carrying a 5lb + pack on your back full of gear. In backcountry this can be a lot heavier with avy gear, but I'm mainly doing "fitness laps" at the resort and use the same running vest. At UTMB. you'd be lucky to get your pack down to 4lbs... The logistics of gear and gear changes in skimo make gear in ultra-mountain running simplified. You dial in hydration and nutrition very well also!
4. There is a lot of sponsor over-lap with gear and promotion. A lot of companies like COROS or Spring Energy or Strava or Vest/Hydration pack companies love outdoor industry sports in general. Some mountain runners in Europe have a main sponsor in a brand like Dynafit etc. (even for their summer mountain running season).
The Euro attitude of "mountain athlete culture and experience" ties these disciplines together a lot better. Of course since "Skimo" is so niche in the US it's really only in the mountain access towns where people experience these things in the US....and being wealthy enough to buy all the gear and the time and energy to get on a resort or into the backcountry.
Being able to spike your heart rate and push the legs and lungs at high altitude mountain running trails I'd be running on in the summer anyway is pretty fun...especially when everything is covered in snow!