The biggest problem in differentiating is what they train in. I tried to stick to people at ages where they would still be able to be viable World Cup athletes (looking at results for people below 35, for a start, trying to pick out people who are specialised Ski Classics competitors rather than those who have moved across after their World Cup days are over as, as you mention, there's a fair amount of 'retirement home' demographics there). And that there are so few 30-50km races on the World Cup compared to 10-15km races that just the numbers of results at each distance alone doesn't tell the whole story. Those entries in the Holmenkollen 30 along with Falun and Sochi are the only World Cup level races of that distance that Boner did, and by the time of the latter ones of those she was very much a part time World Cup entrant who specialised in coming in for those longer races as well as the home events in Switzerland. Another possible factor for her is altitude as well, as I note 3/4 of those top 15s coming in Toblach and Davos. The other is at an event in Szklarska Poręba that none of the Norwegians, Swedes, Finns or American A teams entered. Curiously she also only entered the Holmenkollen 30 when it was in freestyle, whereas her top results in the 30k, at Sochi and Falun, were both in Classic. And also at championships races where the number of entrants from the biggest teams would be limited of course.Some interesting examples here, but I’m not sure if they really work as athletes being clearly more competitive over 30-50km than for example 10-15 km. Boner never broke into top 20 in Holmenkollen 30 km for example, and she was top 15 4 times in the World Cup, all races of 10-15 km. In the World Champs she also had a top 10 in the 10km once.
Ishida is a good example of a very strong 30 km skier, but she also had plenty of top 10 and even top 5s in 10 and 15 kms, so not really a clear difference saying that she was so much better in the longer races.
Smedaas was even a decent sprinter, she’s won sprint races in the Norwegian Cup and she’s been skiing sprint races for Norway in the World Cup. All her best WC results are from 10/15 km as well.
Persson is another vague example, he has some decent results in sprint, I think even last year he won a sprint race in the Swedish Cup ahead of Oskar Svensson.
In general I think there are almost no differences in the skill set that is needed to be good in a 15 km interval start and a 50 km mass start until the final kilometre.
And I think that there is a lot of proof that the skill set needed for sprint race are also very compatible with the long races in Ski Classics. A nice remembering that even sprint races and sprint athletes are endurance events.
I think, moving back to the sprint as a general thing rather than Boner specifically, the other difference is that between the sprint quali and that of being successful in the sprint as a whole, with four times around the course required and the short recovery time required for that. It's the big problem in integrating it into the Tour, because practically nobody bar a couple of athletes in each gender is going to have done the 'full' distance of the race, in qualifying for the finals each time.
However, the random factor in sprints - getting taken out by an also-ran, a broken pole on the run-in or just being in the wrong heat - can end up being far more impactful due to the bonus system, and I dislike both that the system gives the biggest benefits to the format I value least, partly as it is the most susceptible to chance, and that increases the artificiality of the effect on the GC - as mentioned before, Klæbo actually set a slower QF time than Amundsen, but Amundsen wasn't in the top 2 so relied on lucky loser times and got eliminated, Klæbo was in a slower heat he could get in the top 2 of and went through, and took 52" of additional bonuses as a result. A case can be made against my personal valuation of the format, sure, but I don't think a case can be made against it being the most luck-dependent individual format.
I don't think you can solve that in a way that satisfies everybody, because you would have to reward getting further in the competition to justify taking the start. But as the gaps elsewhere in the race are being reduced down to nothing, the sprint time bonuses are becoming increasingly impactful in setting the GC. Much like how in the late 2000s stage win bonuses in the Grand Tours were 20", 12" and 8" and now they are 10", 6" and 4", I think we're overdue a reduction in the amount of time bonuses those stages give.