Observational studies like these are useful for guiding future research, but, on their own, they're essentially useless for informing lifestyle changes.
Natto-serrazime is probably an excellent complement as it is on the other side and is a dissolver. (Noteworthy: Pterostilbene + Glucosamine similar to EPA reduces fibrosis)
The interesting connection is how this is needed when we are older, but not younger. When younger ERa activates more which does this all on its own. This is the connection to why 2/3 of alzheimer's are post-menopausal women and why HRT is important.
Edit: and to tie this to APOE as it is the gene most associated with Alzheimer's. e4/e4 requires more choline so someone with e4/e4 is more likely to be choline deficient. EPA/DHA usually attach to Phosphatidylcholine (PC) when in the blood/brain. PEMT is a gene controlled by ERa to make choline, but from the above less ERa activation and we make less PEMT so less choline and less PC. Choline is the precursor to Acetylcholine (primary neurotransmitter for memory and focus and essential for REM sleep). This is why Choline is known to help with Alzheimer's.
*edited: %-points instead of %
EDIT: don't assume the causality
That 0.08% reduction would mean approximately 28,000 fewer EOD cases - not to be sniffed at!
And to be clear, many things that people worry about is less likely than that. Homicides (over an 8 year period about about 0.04 per 1000 people), terrorism (vanishingly small), and on and on.
None of this means that people should stock up on omega-3s, and as likely the study is actually finding a correlation with something else (e.g. wealthier people enjoy more fish rich diets and are less exposed to toxins, or something else), but halving something terrifying that isn't that uncommon is legitimately newsworthy.
However, the estimates are still probably overestimated. Confounding, p-hacking, publication bias, all move us towards larger estimates.