The researchers cautioned, however, that they cannot draw a direct, causal connection between meditation and preserving gray matter in the brain. Too many other factors may come into play, including lifestyle choices, personality traits, and genetic brain differences.
Good reporting.
However even if meditating doesn't have a direct effect on the brain (it is in some forms just sitting there) it encourages the type of self-reflection that leads to healthier choices.
The moments where you go "Oh, activity X is unhealthy and makes me unhappy so I should stop" cause a healthier more balanced lifestyle. So there is a completely non-woo mechanism for why meditation could be improving long term health.
Also noteworthy is meditation is a practice that's thousands of years old and there are many different types of it and just because one of them is beneficial doesn't mean they all are.
This is a small sample, and it's just as plausible that (I'm making this up) vegetarians are more likely to meditate, and a vegetarian diet correlates with more grey matter.
If this is a significant result, the takeaway isn't that "I should meditate to keep my grey matter", but rather "I should be the kind of person that would have meditated in this study if I want to keep my grey matter".