> But my understanding is non-DHA is not helpful so I may not be understanding correctly

A lot of the common wisdoms about fish oil and omega-3 were based on early studies that had some too good to be true results. As studies were scaled up to more participants and more rigorous methods many of the amazing early results gave way to less impressive or even null results.

I think it’s a good idea to get a mix of omega-3s in your diet, but given everything I’ve seen I don’t think it’s all that important to start micro optimizing everything with isolated supplements. Consume a mixed variety of sources and try to get some fish in there every once while.

The importance of DHA specifically in this study is a good example of how individual studies don’t tell the whole story. This could be a spurious result that is correlated with something else that leads to elevated DHA in the blood (diet choices). Supplementing isolated DHA could miss whatever the real factor was.