The major difference between the failures of Matthews/Marner and that of Nylander is that while Matthews and Marner simply are unable to perform, Nylander fails to exert effort.
It's by far the simplest and easiest form of improvement possible - actually try.
There is absolutely no reason the best skater on the team cannot keep up with Dylan Cozens when he has a head start, and yet he is beaten by 20 feet from blue line to blue line, and on top of that, coasts from his own blue line to the net.
I mean, it's entirely possible that even if he put in 100% effort, Nylander wouldn't have been able to make a difference on his own, and maybe that SHG would still have gone in, but if you don't even try, you don't give yourself the chance to fail due to performance.