Does just doing grip strength exercises cause people to live longer? Or is the correlation a result of the fact that activities that increase grip strength increase longevity (or are only able to be done by people who are very healthy)?
What I do know, though, is that my grip strength is fantastic, even though I don't squeeze any spring-loaded thingies. It's a side effect of doing regular deadlift exercises, as part of my strength training (roughly based on 5x5).
I also don't find it hard to believe that my strength training does bring benefits. Some are certain: life is much better when you're strong. Some are less certain, like living longer.
Watching TV all day on the couch drinking beer and potato chips - but with a grip exerciser in your hand is probably not going to give you much of a benefit.