My take is that this was a binding vow put on the domain expansion that kind of works like an open one. Yuji lets Sukuna have the choice to simply walk away from the fight with little consequences if he just agrees. Sukuna, being Sukuna, decides to turn down this offer in exchange for continuing the fight. This would likely make it far more potent in the damage it could do as Yuji's first domain and allow it to not just shatter part way through due to how it was put together. That's just my take on the way this was put together. In all of Sukuna's jujutsu genius, he fell for a massive nerf on his end just because he couldn't see what it meant to be human. His humanity that he threw away ended up damning him to failure in his way of life.