The intend of the study was to measure the 'hit' that is so often talked about in golf. They were surprised to learn that with pros there was no 'hit'.
They did find increasing pressure on that part of the grip with amateurs. They did not attempt to identify the source (e.g. arm or wrist or other action).
Based on other golf writing (but not studies) I think the source of the pressure was trail arm extension and/or trail wrist straightening. I don't think insufficient bend alone would have caused it if it had been maintained however the force of the club on an insufficiently bent trail wrist could have caused a reflex reaction to cause the wrist to straighten due to proprioceptors kicking in.