The last paragraph of the synopsis above kinda details why this might matter - areas of the brain don’t have such defined and limited purposes as musculature.
A recent similar study found that the area of the brain responsible for spatial location is smaller in people who rely on sat-nav exclusively. That might not seem like much of a problem, as everyone has one in their pocket now, so so what? Well, it turns out that that area is also responsible for mapping our social landscape too, and if it shrinks, our ability to maintain larger and more complex social groups suffers. The ability to read is quite a specific skill, but strongly linked to the kind of imagination used to create these complex worlds and characters in your imagination - how would loosing that ability affect designers/artists/architects etc?
Ha, yeah, to be fair, I didn’t read the article, I just looked at the research paper it was based on. I’ve given up on a lot of science reporting, it sensationalises things from otherwise sensible studies far too often