Yes, he has, as anyone with access to stats can tell. Look at the data on his shot selection in the playoffs, simply becomes more of a peripheral player, which matches the eye test. And no, Game 1 is not the same as Game 7.
Every playoff game is worth the same, and have their own unique pressures. If Nylander did better in games 1-4, we wouldn't need games 5-7 in the first place. One cannot just pick out a minority of arbitrary games, and declare the scoring leader in those specific games to be the better player. Especially when the sample is meaningless. You can't claim game importance when you're excluding elimination games in favour of random game 5s, and you can't claim scoring difficulty when you're including game 5s and 6s that historically have no difference in scoring levels relative to earlier games.
With access to stats, we can tell that many of these claims are wrong. We can see that his shot selection is fine. We can see that some took a stat about Marner's shot distance increasing in the playoffs one year, and started making incorrect assumptions about him being a peripheral player. When it was actually just a result of two mundane things. A dump in from neutral ice counting and skewing the numbers, and two extra shots than average from the blueline, because we ran a set play against Vasilevsky's weakness. Those extra shots were goals, for the record. Just goes to show how poorly a stat can be interpreted, when you only make assumptions from the surface.
Similarly, the actual difference within elimination games remains negligible. Marner's underlying stats remain strong, and he contributes to the 2nd most goals and 2nd fewest goals against among our core 4. The production difference is a whole 3 points. Negligible to begin with, and even more-so when you realize that the difference is primarily just Marner picking up a bit less credit on the goals he contributed to in this particular sample, while playing better defensively in tougher matchups.
Marner has been the better player in both the regular season and playoffs, and there is really zero argument for his contract to be less than Nylander's.