One possibility. But the original study carefully controlled for exercise and lifestyle factors. To corroborative the causal relationship, we also have extensive experimental evidence where UV and/or bright light on skin/eyes directly improves systematic biomarkers associated with longevity[1][2][3]. These improvements occurs independent of vitamin D synthesis.[4]

[1]https://www.sciencedirect.com/science/article/pii/S0022202X1... [2]https://www.sciencedirect.com/science/article/abs/pii/S03785... [3]https://www.hindawi.com/journals/bmri/2016/1934518/ [4]https://academic.oup.com/endo/article/159/5/1992/4931051?log...