Is it possible that only people with larger hippcampi and other cortical regions are good at memorizing large texts? (that is, the causal link is inverted)
Is it possible that people who learn or do something focussedly over a long stretch of time (practice music 4 hours a day) show larger development over those same areas than the average distracted person?
It's more than possible -- it's well-documented with some kinds of meditative practice. [1]
The only interesting question this article raises for a knowledgeable person is whether or not the Sanskrit language _itself_ would have an effect on cognition, but it never succeeds to prove it would or offer a mechanism for how it would work, and instead asks for funds for more studies.
[1] https://www.reddit.com/r/meditationpapers/top/?sort=top&t=al...
https://www.reddit.com/r/MeditationNerds/top/?sort=top&t=all