I agree it sounds more like an opinion rather than based on a proper scientific study. Since ALA is a precursor for DHA even if only creating limited amounts; consume enough of it then you'll make more than enough DHA. From this pubmed study I can see flaxseed/walnut oil is going to provide enough ALA for this purpose https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2989356/.