Unless those cave walls could be neatly concealed in a pocket or under a desk and served up a stream of rapidly-changing Skinner-optimised content, accompanied by irregular purposefully distracting notification signals, I don't think this is a good analogy. Why is it so hard to believe that we are getting better at hacking our attention spans?

Indeed, the cave wall comparison highlights the difference between stimulation and distraction. Daydreaming while staring at the walls of Lascaux would be a significantly more useful activity than having your attention micro-partitioned by the demands of a Snap streak.