[O.B.Left]

I don't understand why it can not be the clubface as well, since the sweetspot always is in the middle of the clubface. What am I missing?

Hmmmm .........yes the sweetspot (the opposite end of the sweetspot plane from the #3pp) travels the inclined plane but the face angle's relationship to the Inclined Plane varies for all but the impact interval of Angled Hinging. For instance, the properly aligned clubface is not necessarily on or parallel to the inclined plane at top (or parallel to the left arm for that matter). The left hand lays on the plane at top but not the face, necessarily.