Thanks for the feedback: will update the second link to point to the correct source, and I think "isolated findings" would be a more accurate word here.

I agree that many of the studies are about cognitive decline, but there's also research on improving cognitive function, which would probably be closer to "brain boosting" category you mention.

For example, from the first source I listed, "Six trials (including 1757 participants) assessed executive function, 5 trials (including 1426 participants) assessed effects of LCn3 on processing speed, and 11 (including 5698 participants) assessed memory. Meta-analysis suggested little or no effect for all of these measures (as well as the subcategories of memory, all moderate- or low-quality evidence; see Supplementary Material for further information)."

"Current evidence supports the finding that omega-3 PUFAs with EPA ≥ 60% at a dosage of ≤1 g/d would have beneficial effects on depression." https://www.nature.com/articles/s41398-019-0515-5

Is this considered brain boosting?

I don't think so, I would consider "brain boosting" something that makes the brain of an healthy person perform better at certain tasks, not something that improves it for people with a disease.