Somebody needs to read some Kuhn :-P
Vitamin D deficiency correlates strongly with time spent sitting around indoors. They control for physical activity, but not for time spent indoors as far as I can tell from the linked article. Separating time indoors from vitamin D levels would seem to be a tricky endeavor, so it's understandable that that's left uncontrolled.
A vitamin D-depression connection would make some sense. Given that humans largely evolved spending much more time outdoors than modern humans, deleterious effects of indoor life would not be surprising. +75% sounds like an extremely strong effect size deserving of some skepticism.
The original source: https://tilda.tcd.ie/news-events/2018/1813-vitd-depression/
You can take a vitamin D supplement and stay indoors or otherwise keep your routine and still receive the hormonal benefits if you are deficient.
There really needs to be a lot more discussion over the specific scientific realms with lots more independent commentary on the study and results.
And as comes up frequently in human psychological issues repeatability is the gold standard and there isn't any in this specific case.
And humans and their behaviors are so complicated and so difficult to quantify that it seems so unlikely that one specific substance would be a "miracle cure" for human depression.