But there isn't just a correlation. There's also a fascinating idea: better grip strength might help prevent a common cause of death in old age: falls. I've never made that connection before, but it seems very reasonable now I think of it, and compelling in its implications. That's good enough for me to want to discuss it, maybe even to motivate me to work on my grip strength.
If someone ever brings me evidence that having stronger grip does not help you avoid falls, then (a) that would be surprising and interesting, and (b) I wouldn't feel like a fool for having improved my grip strength anyway.
Empirically proving causation is often difficult or impossible. It's irrational to expect every potentially useful idea to meet that high bar before taking it into consideration. If someone is trying to sell me something, or if the idea goes against common sense, I'll demand proof, but that's not the case here.