Casselsten is basically the same age as Willander and significantly outscored Willander scoring at basically a point per game in 19 games compared to Willander's 25 points in 39 games. Casselsten is also undrafted. The other player, Engstrom, was a year older and draft, but only played 7 games so had very little affect on Willander's production.
The following are the defensemen that outscored Willander in the J20 last year, and their age and draft status as per Elite Prospects:
Lagerberg Hoen: 18 – Undrafted;
Axel Sandin-Pellika – 18, Drafted
Anton Davidsson – 19, Undrafted
Samuel Eklund – 19, undrafted
Rasmus Larsson - 19, drafted
Stefan Milosevic – 19, undrafted
Loke Södergren – 19, undrafted
Zétény Hadobás – 19, undrafted
Simon Forsmark – 19, undrafted
Simon Lindblom – 19, undrafted
Dennis Good Bogg – 18, undrafted
Alexander Fredriksson – 18, undrafted
Gustav Hedberg – 18, undrafted;
Leon Matthiasson – 19, undrafted;
No real surprise that ASP outscored Willander, but then you have 4 undrafted players the same age as Willander that outscored him , and 8 undrafted 19 year olds that outscored Willander. So 8 guys were 1 year older, but weren't even drafted, so I think its pretty fair to say that a top ten talent like Willander could out score unddrafted players that were only a year older.
I'm not saying that Willander is a bad prospect, but his production was poor last year based on where he was drafted. Frankly, I don't think Willander was ever drafted based on his offensive capabilities like ASP for example, so I am not too worried. But I think criticism around his productio is fair at this point.