This is an interesting perspective and, if I understand you correctly suggests that players CAN become better playoff performers even if they were at one point considered not be strong in the post season. Your theory is that these players did it because they had coaches that allowed or encouraged it through their systems and approach.
It's not JUST that.
Jonathan Marchessault isn't some bruising big-hitting grinding player who wins all puck battles.
He won a Conn Smyth by being a slippery player who could get behind defenses and capitalize on open looks. His coaches designed plays to get him into open areas of the ice to rip it past the goalie in prime locations.
Marchessault spilled the beans on the team strategy against the Oilers:
Are Skinner, Nurse and Ceci getting too much blame? Definitely
edmontonjournal.com
When I watch Mitch Marner being asked to take a massive hit to make a defensive play, or win board battles, or how he's never given time or space and asked by his coach to battle through it, I think its a massive disservice to the skills that MM actually brings and doesn't play to his strengths at all. It plays completely to his weaknesses, and snufffs his strengths.
That's just terrible coaching.
The problem for the Leafs, is I don't think Berube is any different than Keefe. He's a rigid systems coach who forces other teams into playing into HIS system, rather than an adapt.