Based on your logic, Marner was underpaid compared to Kane, because the gap between 94 points and 72 points is bigger than the cost gap. But players don't get paid based on just one season, and I showed you how Marner's entire pre-signing period compares. 6th best in the entire cap era (for which he received the 10th biggest post-ELC contract), and consistent with your chosen comparable.
Marner had 55 more ES points at time of signing. He had 0.631 ES points per game compared to Kane's 0.516, and 2.56 ES points per 60 compared to Kane's 2.12. But what I actually showed you was every single game state. A 20.6% bigger contract for 20.8% higher ES production, 25.5% higher PP production, plus PKing and an extra year.
He was a better PP player. He had 1 less PP point (69 compared to 70). The difference is, Marner did it in 575 PP minutes, and Kane did it in 740 PP minutes, meaning Marner produced better on the PP (7.19 > 5.67).
If you want to adjust for league scoring levels across eras, that would mean a 20.6% bigger contract for 12.8% higher ES production, 21.0% higher PP production, plus PKing and an extra year. Still reasonable.