In fairness Johansen and Arnekleiv were both decent juniors, but nothing that would suggest this was possible. It's not uncommon since they don't contest the Junior World Cup for Norwegian skiers to disappear a bit between their junior days and hitting the World Cup seemingly fully-formed, or athletes who were in the World Cup team reaching around 25-26 before they hit peak performance levels (see for example Tora Berger, who had two anomalous podiums - a sprint-pursuit double 2nd - until her first win at 25 years and 11 months - an age at which, for example, Magdalena Neuner and Laura Dahlmeier had both already retired). Skogan's Individual was shockingly good as she has never really stood out at any level and reached her mid-20s but it was built on strong shooting and she was barely scraping into the points today.
Knotten is the weirder one, she's more established than either MKJ or Arnekleiv, but she has been around a few years as one of those "slower athletes you hide on leg 1 of the relay" and reached 28 years of age before she has suddenly leapt up in skiing level this past off-season. It's not as marked as, say, Synnøve Solemdal going from being around 40th in ski time to outskiing Magdalena Neuner over the 2011 offseason, or Marte Olsbu going from being a "shoot first, ski second" athlete to becoming one of the fastest in the world over the course of about three weeks ahead of a home World Championships, but it is a marked improvement nonetheless.
What is worth noting however is that for the first time this year, the team went with a fully meritocratic selection process from the Sjusjøen events, without having athletes who were pre-selected, as with no Eckhoff and no Røiseland, plus Lien injured, they were basically left with nobody other than Tandrevold who would be available for Östersund who had auto-qualified anyway. There has been some criticism of Skogan making the World Cup over the likes of Kirkeeide who are on the IBU Cup, more promising and several years younger, plus people like Erdal and Kalkenberg who are, if unspectacular, at least more reliable and proven at a similar age, but that's what happens when you say specifically that one weekend's racing will select the competitors. The other thing is that it means people on the fringes of selection have to hit peak form at that point, so they may well have been peaking for those. After all, at those Sjusjøen races the German team were absolutely obliterated, but two weeks later they appear to be hitting the ground running too, so we might reasonably anticipate that Johansen and Arnekleiv will tail away somewhat - it's just figuring out what kind of level they will be at when they regress to whatever their new mean is. Knotten is far more experienced and I think her base level will likely still be relatively high. After all, a few years ago we had Synnøve Solemdal wearing the yellow bib for a race or two early in the season only to quickly revert to her latter career struggles, and Ann-Kristin Flatland managed an early season win in 2013-14 before never being remotely close to that level again all season - but at the same time, Tiril Eckhoff was very consistently just below the top tier level for the whole 2013-14 season, being bottom of the top 10 of the World Cup overall but with hardly any podium results; she rocked up to 2014-15 - so a similar age to where Arnekleiv is now - skiing at a considerably stronger level than before, but she maintained that improved level all the way to her retirement last year.