"So, once the bucket starts rotating, the surface of the water will immediately begin to change."
"Then the spin angular momentum of the central mass and the orbital angular momentum of the surrounding mass will conform to a certain function, that is, the total rotational inertia of all matter in the universe sums to zero."
Anyone who's taken graduate level physics should know this.
Xu Yun's line of explanation is the very beginner-friendly concept of intrinsic reference points.
That is, he defines the reference frame inside the boundary of the target region as the intrinsic reference frame for that zone, where all spaces at rest with respect to this frame are set as the intrinsic reference points.
These intrinsic reference points move collectively as a whole, experiencing identical accelerations from all external forces.
Therefore, the positions between intrinsic reference points don't change relative to each other.
