They quote an article that says
cats need
in their diet linoleic and arachidonic acids. Now linoleic acid is found
in flax, but cats, according to this website, can't change that into arachidonic acid.
Putting fish oil on sound like the smarter way to go, basing my opinion formulated from what I read, when it comes to topical, such as the kind you talk about, because it arachidonic acid in it.
They do caution too much Omega 6 is not good for cats with chronic kidney disease.