Did not read the paper but did not find any reference to omega 6 nor the ratio of omega 6 to 3. There are studies showing that the ratio is more important than the amount of omega 3 [1]. Because humans typically eat 16:1 (6 to 3) increasing 3 is actually just making the ratio more even which is what matters. A good food for this is flaxseed which is 1:3.5 which is less than fish which can be 1:30 or higher but does not come with the problems of consuming fish. In this reference frame, one can think of avoiding bad ratio foods instead of eating fish. Turns out that basically the most common oils and red meat have ratios of 20:1 (the wrong way) and so just avoiding oil and red meat ends up accomplishing the same goal. As always, nutrition causality is hard to establish, but trying to cut way back on oils and red meat is going to pay much larger dividends than eating more fish.
[1] - https://pubmed.ncbi.nlm.nih.gov/12442909/