I'm with Kyper on this one, it doesn't work that way.
There is no chance - zero - that Treliving has or will communicate to Marner that he's waiting to see how the playoffs go before they decide what to do with him. Because if he took that attitude, Marner would rightly say he's walking on July 1st, period.
His argument would be that Treliving didn't hold off on Nylander's deal until last year's playoffs, and it was pretty clear Auston was getting extended with no such conditions. So Mitch would simply ask "why play hardball with me?" And he has a point.
As a fan, I absolutely would want to see how things go in the playoffs before extending him - if they fail again, something drastic must be done. But I wish they took that approach years ago, or told Mitch when they re-signed Willie that he's their guy and moved Marner when they had their chance.
The Leafs didn't so here we are. And my belief is that MLSE's 100% intention is to re-sign him after the season is over, regardless of how the playoffs go, unless Mitch rejects every offer they throw at him and walks.