Remember that time Stalberg as I believe a rookie led the league in goals after the pre-season? I wouldn't project too much based on a small sample size training camp rosters.
Stalberg didn't lead the league in goals during the pre-season
after a season of leading the Leafs in regular season points and almost doubling everyone else in playoffs points.
I get the caution against recency bias, but Mitch Marner is as good an example that we have in which any recent positive indication is consistent with conclusions that state Marner is an optimal performer now and will be going forward. The only question is - which I think the OP is asking - how much better can Marner be?
If we look at his junior numbers and pair them with his professional arc...It's not an unreasonable question being posed. And in fact, I think we're replicating the kind of parity within the lineup (at least at the forward position) that Detroit in the 90s/00s had. For some, Yzerman was the best RedWing and for others - and with valid reasoning - it was Sergei Fedorov.
I'm not arguing against Matthews being the best player on the club anymore than I've argued he shouldn't be the captain of the club in the near future. But...I have stated that I could see Marner as the captain, and per the OP, I can see how some might believe Marner to be the best player on the club.
For sanity's sake, I'll leave what the contractual entailment might be until next summer.