There's a limit to the number of contracts a team can have and I'm not sure why a team would use one of its six remaining contract slots on this player.
The Senators only had 5 picks in the 2023 draft. Most of those were in the 7th round. Their earliest pick was in the 4th round. Additionally, they do not have a deep prospect pool. For a team that has come out of a long rebuild, they did not accrue the same level of prospects that most other teams do. They averaged about 7 picks per draft since the rebuild started in 2018. So it isn't as if the rebuild allowed them to restock the cupboards by all that much. They did an exceptional job at drafting a core of top-end players like Stutzle, Sanderson, Tkachuk, etc - but did not build up as much organizational depth as would usually be expected in a successful rebuild.
This is because they were unable to capitalize on their cap space due to the combination of a restrictive budget, and an owner who needed cash flow through ticket sales. Meaning, they couldn't go scorched earth. They had to make trades like a 2nd round pick for Derek Stepan, or make moves like holding on to Mark Stone and Matt Duchene throughout the season rather than trading them at the draft in their contract year, when they likely would have commanded the biggest return.
The short of it is, they are unlikely to have issues with their contract limit during the 3 year period of this contract because they simply don't have a higher-than-normal amount of players who will come from the reserve list and command ELCs. The Senators have nobody on their reserve list to sign until 2025.
On a side note, this is not a random signing. Toure played in Carleton-Place and Hawkesbury. Basically, in the Senators backyard. Who knows what his Junior career might have looked like if things weren't so fragmented due to COVID. Had the 2020-21 season not been cancelled, perhaps he would have ended up in the CHL earlier, and by extension maybe he ends up drafted. He is a late bloomer. It's a free 7th round pick basically.