While the headline is also true, the significance of this study (as the article explains) is showing that supplementation can reduce mortality. It was previously known that low blood levels of Vitamin D were linked to negative outcomes, but it was much less well agreed whether supplementing with Vitamin D would improve outcomes.

It's an obvious hypothesis, of course (low X associated with negative outcomes, so increasing X will improve outcomes), but not necessarily guaranteed: the low Vitamin D levels could've been symptoms of a different problem that isn't fixed by just supplementing with Vitamin D; or the supplements might've failed in various ways to be taken up or activate the right mechanisms.