A soul contract is anchored in the soul itself. As long as one does not release it, no matter how great the opponent's abilities are, it can never be undone.
Just like the Immortal Emperor, Little Yellow Chicken, who was previously only a God King while the other was an Emperor, he was similarly helpless against such an oath.
To prevent either party from going back on their word and suffering a backlash, the Master Teacher Continent had once specifically established that even if a party could break free from the Book of Dao, they could never shake off a soul contract!
Chapter 916