Paul Bissonnette goes on to explain that the Toronto Maple Leafs should make Mitch Marner earn his next contract whether that is with the Maple Leafs or elsewhere. He also mentions that this is assuming Mitch Marner envokes his no-trade clause. This likely means Paul Bissonnette would be open to trading Mitch Marner but don't let the player control the situation.
This would be a risky move for the Toronto Maple Leafs that could pay off or backfire. The best-case scenario is Mitch Marner comes in and plays lights out leading the Maple Leafs to a Stanley Cup. At that point, without an extension, his price would go up significantly.
In a worst-case scenario, Mitch Marner comes back and struggles and you end up trading him at the trade deadline for a discount or let him walk for free. The risk of making this mistake might not be worth the reward that keeping Mitch Marner might possibly bring.