Nutrition studies are notorious for problems with self-reported data, correlation without causation, and even cherry picking so …
‘We found that higher beef intake may be protective against major depressive disorder,’ they added.
and Vegetarianism May Be Linked To Depression, Study Suggests (forbes.com)
I (and the researchers) can think of reasons this might not have to do with the specific foods consumed.
The research on vegetarian eating and its effects sometimes finds vegetarians are healthier versus omnivores, and vice versa.
Most vegetarians eventually resume eating some meat, for a variety of reasons:
84% of Vegetarians and Vegans Return to Meat. Why? | Psychology Today
As they note, meat reduction is a more successful strategy for most people, versus total meat elimination. I have meat once or twice per week – even though I grew up in a family that had meat every day.