This reminds me of the recurring pattern with every new medium: Socrates worried writing would destroy memory, Gutenberg's critics feared for contemplation, novels were "brain softening," TV was the "idiot box." That said, I'm not sure "they've always been wrong before" proves they're wrong now. Where I'm skeptical of this study: - 54 participants, only 18 in the critical 4th session - 4 months is barely enough time…
Soapbox time. They were arguably right. Pre literate peole could memorise vast texts (Homer's work, Australian Aboriginal songlines). Pre Gutenberg, memorising reasonably large texts was common. See, e.g. the book Memory Craft. We're becoming increasingly like the Wall E people, too lazy and stupid to do anything without our machines doing it for us, as we offload increasing amounts onto them. And it's not even that…
It seems more likely that there were only a handful of people who could. There still are a handful of people who can, and they are probably even better than in the olden times [1] (for example because there are simply more people now than back then.)
[1] https://oberlinreview.org/35413/news/35413/ (random first link from Google)