The sources mentioned in that article are ALA, which have conversion rates to the desired DHA/EPA that vary greatly between individuals [1]. Typically, something like 5% of ALA is converted to EPA and even less is converted to DHA. It's better to eat the preformed DHA/EPA rather than relying on the conversion.

edit: Harvard health seems to even disagree with themselves: http://www.health.harvard.edu/staying-healthy/why-not-flaxse...

[1] https://www.ncbi.nlm.nih.gov/pubmed/9637947