He was not. I've provided a lot of comparisons, including one that you just responded to, but the problem with saying show me a direct comparable with a similar pre-signing period and X contract with X term is that that doesn't really exist, especially recently among wingers. Not because Marner is overpaid, but because Marner as a player is quite unique even within the rare group of high-end young forwards in the cap era, and both Marner's pre-signing period and contract fall in a bit of a dead zone - not quite at the top top tier of a handful of generational players, but pretty easily a clear step above pretty much everybody else, with Rantanen being the only winger even remotely close to come along in a decade.
The closest contracts to Marner are Vanek and Heatley, but I assume you don't want me doing that comparison, because it would just make Marner look good, and I'm sure you'll argue that Vanek was an offer sheet and Heatley was so long ago. We could look at Kovalchuk and Nash, who got bigger contracts than Marner too, but same thing, right? Just makes Marner look good. So we step down a tier and look at the likes of Rantanen/Kane, which I've shown a number of times, but people refuse to accept the clear and obvious gap that existed between them and Marner that's consistent with their contracts, because they don't think Marner's lead in P/GP over their ELCs is enough, and they refuse to acknowledge the impacts of PP time (which those players got pumped full of) on that raw production, that's inflating the perception of their relative quality.
So I'm open to looking at whatever comparisons you want, but it's got to come with an acceptance that raw points don't always tell the full story of offensive impact, and like anybody else in his position, Marner was never going to and should not be expected to not get compensated appropriately relative to his offensive impact just because he came up in an era of low special teams time, on a team that got abnormally low special teams time, under a coach that intentionally tried to neuter his production as he mentally abused him.