I wouldn't be so sure if I were you.
I don't think that's true either and I don't like labelling players "good playoff player" or "bad playoff player", it's never that simple.
What I would say is this - during the regular season, M&M have both been much better than Nylander but in the playoffs over say the last 3 years, that gap shrinks considerably. Maybe the massive pay gap between these guys is justified re. Nylander during the regular season, for a guy like me who cares a lot more about the playoffs then record breaking Novembers etc., the massive pay gap is not justified.
Based on performance in the last 3 playoffs, I'd probably go something like this:
8 million - Nylander
9 million - Marner
10 million - Matthews
Based on expectations which are in turn based on their cap hits, Nylander has earned his money and more while M&M have underperformed expectations by a 2-3 million each and that's why when there is talk of trading Nylander, so many people react by saying why not keep Nylander and trade Marner instead?
JMHO.