Tbh, if it is true at all that people are more precise with their fingers, it sounds like just learning and adapting on a physiological level, not a genetic one. If you lift a lot of weights, you will we more muscular, but that's not genetics necessarily. If you had an identical twin who didn't do that, they wouldn't be as muscular.

Also, that story of evolution sounds kinda...larmarckian? Of course us using our finger more during our lifetime doesn't necessarily make future generations automatically better with their fingers, right? But maybe I read your description wrong

Also the obvious thing...is it true that humans now are more dextrous with their fingers than at most points during history? I can think of a lot of things before smartphones that also require a lot of dexterity