I think they are overlooking an obvious explanation from our arboreal past in these. When hanging out in trees, or climbing in other ways, it is beneficial to clench the hands when startled, stressed, or anxious to keep from falling. And with the predominance of right-handedness in humans, an instinctive clinching the branch with the left hand would leave the dominant hand available to deal with the threat/situation. Clenching would have been beneficial in other ways in our past as well. Lose your grip on an animal you have caught and you might not eat, we do not have claws or fangs with which to snag our prey like other predators do. Lose your grip on your hand-held weapon when facing a dangerous animal or an enemy and you might lose your life. Just because over 99% of humanity has been removed from such realities in recent centuries by technology does not mean we have lost that reflex when we get stressed and our more "primitive" nature kicks in, hundreds to thousands of millennia of evolutionary programming is not so easily overruled.
21st Century American cubicle warrior equivalent: Lose your grip on your fishing pole when a decent-sized fish takes the bait and you might watch some expensive equipment disappear under the water.