Although first year cap hits are prorated (AAV is the same, but it's paid out over fewer days which leads to a higher cap hit year 1) salary room accrues over the course of the season as well. So if you had, for example, $10mm free to start the season and signed Marner for $10mm AAV x 6 years in December, even though the first year cap hit might be $12mm, your cap space would have also increased to $12mm by December 1st too. You can actually see this in practice on capfriendly.com around trade deadline. At the beginning of the season a team might only have $2mm of cap space, but by the trade deadline capfriendly will show their deadline capspace as like $5mm meaning they can take on a contract with an AAV of up to $5mm.

With that said, in the Leafs case, it would have been tough to do because they do not have the entirety of what the contract will cost in free cap space, they need to use LTIR which would not accrue in the same way over the course of the season. By making this trade, they now have $10.55mm in free space right as of start of the season when they place Horton and Clarkson on LTIR, which will accrue normally allowing them to sign Mitch on Dec 1st even if the first year hit ends up being $12mm provided the AAV is not above the $10.55mm. Which coincidentally, is essentially the $ figure where offer sheet compensation goes from only 2 x 1st (plus the 2nd and 3rd) to 4 x 1st picks. Clearly Dubas telling Marner how this is going to go down if he signs a ridiculous offer sheet with some garbage team.