The title is a bit sensational; the article directly calls out that they are using grip strength as a proxy for overall fitness and day-to-day ability. So, people who have higher fitness have a lower incidence of depression. I'm not really sure this is news.

It could be that those with poor handgrip strength suffer from a number of health maladies. And though an 'association' occurs between handgrips and depression -- the causality of depression is actually from the underlying co-morbidities.