That's kind of an unfair comment the way you phrased it.
Yes it's true that neither of them have won a Stanley Cup in Toronto, however Matthews has a Calder Trophy on his resume and Marner has won a Memorial Cup/Memorial Cup MVP, OHL Championship/OHL playoff MVP and OHL regular season MVP. So they have won somethings in their careers.
Using your logic other players like Connor McDavid and Leon Draisaitl aren't proven winners since they haven't won a Stanley Cup as well. However they have had personal success such as McDavid winning the Hart Trophy in 2017, Ted Lindsay Award in 2017 and 2018, and the Art Ross in 2017 and 2018. Draisaitl is also the most recent winner of he Art Ross in 2020 and looks to be the favourite for the Hart Trophy this year.
Perhaps I was harsh, but I don't care what they did before the NHL. Some of the Leafs have won as professionals, the Calder in the AHL but again I'm more interested in the NHL results.
Yes, I'm happy Matthews was awarded best 1st. year player. Leafs haven't had much to crow about recently.
And the players involved are marner and Matthews, so not sure how Draisaitl and McDavid come into the discussion, and you could include lots of players in the same boat(never won a Stanley).
Not only have neither won a Stanley Cup, neither has won a single round in the playoffs. Acknowledging it is a team game.
I should emphasize, the main point is they are still very young hopefully with ceiling yet to reach.