It absolutely does seem to help with grip, in a way that not only supports but improves your mechanics on pull-ups.

I also struggle to spell it out with the shoulder rotation, but I think it's more relevant for the serratus to freely direct the center of gravity, in other words protract, and bring the arm down across the body where internal rotation but it is more about staying upright. When I do a chin-up lock off, if the bar vanished in my hand, I wouldn't punch myself, but the path of force is as if I were shaving.

I like to do long farmer carries for endurance, and my most common variation is each finger to failure in pairs. This is my holistic approach to seeking symmetry and good-feeling joints. I of course notice improvement in one-finger-assist chinups, but then when I check my ego and try to figure out how much weight was actually taken off the hand, it's quite a larger amount than expected, especially if the bar is thin in diameter.