Of course, players never evolve. It's a snapshot in time when they are 17/18 and nothing else. Read what you write and see if you actually make sense.
Think the point with Slafkovsky is that he got better as the season wore on, with the tournaments giving him confidence and that even reflecting on his later Liiga play, but hey, the player sucks and will always suck because of that snapshot in time.
Players aren't chameleons over a short span, but hockey is a fractions a second game where confidence plays a large role in execution when hesitation is removed from the equation. If you're skilled, confidence can drastically alter your gamed your production over a small period of time.
At 17/18, players also grow into their bodies and that has a direct effect on their production levels.
In the same vein, players who grow faster than others and can dominate their opponents physically at that level, at that age, can also hit a wall - sometimes temporarily, as they adjust, or permanently, if they can't, once they play against adult players their size with skill at the NHL level.