I think they're referring to sunflower oil's absence of Omega-3s. Per Wikipedia [1]:

> The ratios of omega−6 to omega−3 fatty acids in some common vegetable oils are: canola 2:1, hemp 2–3:1, soybean 7:1, olive 3–13:1, sunflower, flax 1:3, cottonseed (almost no omega−3), peanut (no omega−3), grapeseed oil and corn oil 46:1.

My understanding is that there's some Omega-6:Omega-3 ratio to shoot for, and most western diets are far more biased toward Omega-6, so adding some Omega-3 (sardines) along with a bunch of Omega-6 and no Omega-3 (sunflower oil) would actually work against that goal.

[1] https://en.wikipedia.org/wiki/Omega-3_fatty_acid#Plant_sourc...

I don't think that's what they meant exactly, but rather that the amounts of omega-3 and 6 you consume should be in a specific ratio (or rather within a range of ratios), and the ratio in canned sardines is tilted towards the unhealthy side. There are other pairs of nutrients that follow this pattern as well. I highly recommend reading about them on Examine.com, but I suppose even Healthline will do.

I will add here to clarify my stance on food and health.

There is no healthy food, only food that is healthy for you, and that will be determined by your genetics. Wheat is not healthy for celiac and short chain PUFAs and bad for me.

But too much of any food or nutrient can override genetics as well.

Most sunflower oil isn't really sunflower oil. It's the leftovers being extracted using hexane.