They produced at a shockingly similar level.
They didn't. That's been shown to you multiple times. Evidence of teams considering context is all around us. That's why, for example, we tend to see inflationary impacts on contracts in situations where players are driving their own levels of production in difficult situations (ex. Eichel, Nash, etc.) and deflationary impacts on contracts in situations where players are benefitting from a superior player (ex. Point, Backstrom, etc.).
We also see teams consider the impact of opportunity discrepancies outside of a player's control on raw points all the time. That's why McDavid wasn't counted as just a ~50 point player in his first year, or Heatley as a ~25 point player in his 3rd. That's why Eichel and Nash weren't counted as just ~55 point players when they signed. That's why everybody doesn't have their contracts plummet after shortened seasons. Etc. That's why you're boosting Rantanen's numbers up, in this very discussion, as you refuse to make the same considerations for Marner.
You're making a lot of demands for the most obscure direct comparables and examples of unprecedented situations, because you know that's not what this is about. It takes understanding production metrics, how they are formed, and what they mean. It takes understanding the correlations and driving factors and considerations behind contracts. It takes having an understanding of the history of contracts and where players belong. Marner is right in the cap hit percentage range and ranking range that he belongs.
You want to ignore the elephant in the room, and pretend that the only thing that matters is points.
Marner: 224 points, 10.9m x 6 years
Crosby: 222 points, 14.1m x 5 years
Oh, wait! Not like that! It's actually all about points per game! That's all that matters!
Marner: 0.93 P/GP, 10.9m x 6 years
Nash: 0.62 P/GP, 11.3m x 5 years
Oh, no, no, no! Not like that either! Pretend you didn't see that! It's actually goals! They're super dee duper special because I said so. It's all that matters!
Marner: 67 goals, 10.9m x 6 years
Eichel: 48, 10.9m x 8 years
No wait! Forget that! Goals per game! Final change! That's all that matters!
Matthews: 0.53 G/GP, 11.6m x 5 years
McDavid: 0.36 G/GP, 13.6m x 8 years
You're just picking and choosing whatever suits your argument in the moment, but there's no actual evidence or consistency behind it. Rantanen wasn't even the closest statistically to Marner. If anything, he's closer to an example of compensation being misaligned from raw statistical production because it became disconnected from their impacts. By P/GP, I believe the closest would be Heatley, with a 0.02 difference, and pretty comparable contracts (9.4m x 3 years vs. 10.9m x 6 years).