First off a lot of people are saying Killian is an "ultra mountain guy"....and yes, he is well known for winning UTMB and Hardrock and other 100-milers (Western States even).
However, in my opinion his best performances have actually been from races in the 20-mile to 30 mile range (but technical steep mountain trail courses generally). He is actually really good at events still in the 30-min to 2 hour range.
Now I don't know anything about true mountaineering (Everest or even something like his Matterhorn FKT so I'll leave that out of the picture for now). Obviously he has also been good at super high altitude stuff and has vast climbing experience with ropes/crampons and skis.
But most of Kilians big wins and dominating performances in the realm of MUT have been in "Sky Running" and now "Golden Trail Series" Races. People know UTMB because it is a big deal, but he has been beaten there. He has shown "weakness" (At times) in longer ultras...whereas his dominance in shorter (sub-ultra_ mountain running is probably more prominent. (Granted he almost always gets top 3 in any MUT race he does....even on a "bad day"....except at the UROC 100km in Colorado in 2013 or something like that.....)
I actually think his 2:25 CR at Sierre-Zinal last year was probably one of his best performances ever...consider that that is not an ultra race (it is about 19 miles)...but it is an iconic mountain race because of the climbing and altitude and history. But it is not a technical race. The trail is generally quite smooth and to do well there you really have to have some serious turn-over/ wheels as the final 10-miles trends downhill and there are even dirt road sections on a lot of the course. It's also probably why a 28-min 10km guy like Jono Wyatt had the previous record there. But Jono (great climber that he was and still a Mt. WA course record holder) didn't really venture into longer mountain ultras over 50km all that much.
So Kilian has shown excellent range in all these events, but in my mind it is all just "distance running". Training for skimo in the winter is actually really good prep for mountain running in the summer, much like doing some hill workouts and hilly long runs is great for track and road runners to work on their flat speed. But the opposite can be true to some extent: mountain-ultra runners can benefit from doing some track intervals and flat road tempo runs at times as well.
The heart and lungs don't care what event you are doing if they are operating at the same intensities. It is fine-tuning the neuromuscular system and muscles/muscle build and activation for "Variable Running Economy on variable terrain" that makes these transitions more feasible. People are going to be able to pull that off at different ranges (And some not nearly as good as others!).