Have done some reading on buybacks and to clarify:
1. It's unaffected by this new deal Cirkin has signed; it's a completely separate contract between the clubs only.
2. As a result of 1, even if Spurs do activate it, Cirkin has the final say whether he wants to go back as he is not party to the "buyback contract" between the clubs. If they can't convince him, he stays [EDIT: This might be wrong, different sources say different things. I think it depends on how the deal was structured?]
3. The Sunderland Echo reported that the buyback clause expires on 30 June 2024. No guarantee they're right, but it would make sense. That gives Spurs three years from Dennis signing with us which seems like a fair compromise for all parties involved.
This is a good read, I think people are maybe mixing up first refusals and buy-back clauses;
https://www.mondaq.com/uk/sport/421830/football-transfers-buy-back-clauses-explained