That would be the smart deal to take if you're really that concerned about maximizing the amount of money you can make.
3x$13.5M means he's 29(?) as a UFA in 2027 and likely looking at a near $100M cap. $13.5M would be nearly 16.5% of an 83.5M cap. Which means at a $100M cap the same contract would be $16.5M and no doubt he gets that for 7 years in Free agency if he went to market(Probably more honestly if he's still a Top 3 Center in the game).
Going this route could net him an extra $15-25M in career earnings tbh.
It's crazy he's trying to do this though, at least IMO... It's one thing to be unwilling to take a discount to stay on a long term deal. It's another thing entirely IMO to be unwilling to even take a long teem deal because of the potential for more cap space and more money in a couple years.
That is truly trying to maximize every dime you can get, and you almost never see it to that extent. Tough spot for Toronto if indeed this is what Matthews wants. It basically tells the team he cares way more about the money then the rest of the team and actually winning a cup with that group.