I don't think of plasma sodium concentration as being a good indicator of hydration because it's aggressively homeostatically controlled. As I mentioned, we usually think of sodium shifts as a pathological state. It would be interesting to me if small variations in fluid consumption could also influence sodium concentration to a lesser degree, because it's not really aligned with what I was taught. But I acknowledge that it could be, I'd just like to see data.

The entire premise of the article's interpretation—that hydration status is reflected in sodium concentration—rests on this being true.

Don't get me wrong, the finding that you pointed out (that there is an association between high plasma sodium and aging) is interesting even if the authors' premise is not true! But if small variations in daily water consumption (for example) don't actually change sodium concentration, then the observation could be 100% true and yet their interpretation and proposed follow-up studies could make no sense.

So when I said they dance around whether it is appropriate to be attributing sodium shifts to hydration, that's what I meant.