a combination of the trunk rotation , arm movement and the hand action. They all must be working together correctly.
Actually power comes from muscle contractions. Scott does not say here what muscles are contracting and so there is room for considerable misinterpretation.
In a different post
The correct movement in the down swing is a simultaneous contraction of two major muscle groups. ....
Specifically, these are the pectoralis major and the coracobrachialis on the fromt, and the latissimus, subscapularis and teres major muscles on the back. ...
The three muscles that are responsible for the lateral movement of the lower body during the start of the golf swing are the gluteus medius, gluteus minimus and the tensor fasciae latae. The contractions of these muscles move the lower body to the left side and allow for the drop into the left knee bend.
Note that none of these muscles manipulate the hands. Also note that even this description is not a complete description of power sources by percentatge of power contributed or of energy transmissiom to the club.