You focus on top players which is completely irrelevant. The teams suck(outside 2-3) and the depth suck and that's why every one of those players would have destroyed NLA on their draft year.
I think it's important to note the quality of the players at the top of each league when considering Matthews' stats and its context. It's not like he's getting 3rd line minutes against weaker competition. He plays top 6 minutes against other team's quality players. No coach is stupid enough to try and match against the ZSC Lions' best forward with their 4th line grinders.
Matthews sees the opponent's 1st and 2nd lines, players who are comparable to the top players found within the rest of Europe. The teams at the bottom of the league indeed suck, but within the playoff group those teams would all be of comparable quality to teams in other leagues. Maybe lower end teams in the Liiga, but still within the realm of being able to compete.
The difference between the Liiga and the Swiss League isn't like the difference between the NHL and the AHL, it's probably closer to the difference between a league like the QMJHL and a league like the USHL. Yes, on the whole the QMJHL has more depth per team, but the best players in the USHL would still be among the best players in the Q and we're seeing plenty of top US players drafted high recently.
The Liiga is definitely better top to bottom, but Matthews' scoring totals are rewriting record books. That counts for something in a league with its fair share of ex-NHLers. On the other hand, I don't hear anyone talking down on Puljujarvi and Laine considering they play in a league with the Espoo Blues, Ilves, and HPK and only have modest point totals.
Puljujarvi and Laine are good and likely coming of the boards 2 and 3 (and not necessarily in that order) but I don't know if they'd produce at a similar level to Matthews in the same situation. They're playing depth minutes right now and doing quite well, all things considered, but can they walk in and be the best player on the best team in the Swiss league? Hard to say.
I will say that the Liiga is harder to score in, but whether or not that makes it "better" in terms of quality of team/players is something that cannot be statistically confirmed. For all we know, it could simply come down to quality of goaltending.