Vegan

The most likely contributor towards the plant oils being healthier is, I can only assume, their makeup of Omega 3 and 6 fatty acids. Most research seems to suggest that having a more balanced ratio of omega 3 and 6 fatty acids results in reductions in all cause mortality. Since most diets nowadays significantly over represent omega 6, it's usually recommended to focus on Omega 3 to swing the ratio back the other way.

Butter has very little Omega 3 (a teaspoon only has about 25mg~) and is mostly just a saturated fat, which can be used for energy, but has little health benefits, besides the Vitamin A and E. It does not meaningfully contribute to either Omega 3 or 6.

Seed oils on the other hand can have significant amounts of Omega 3, depending on the oil. There's a bunch of caveats when it comes to seed oils, as how they're processed or the type used in cooking can effect if they are oxidized (and thus become a health hazard). But a high quality seed oil high in Omega 3, such as an expeller pressed Canola oil that was not exposed to too much heat, will offer significant sources of Omega 3 (and 6), without the cholesterol of Butter. I suspect that's where most of the health benefits are coming from.

Since most seed oils are quite sensitive to oxidation when exposed to heat, I personally think Avocado oil is the safest to use when frying, as it has the highest smoke point. But for salads, or low heat applications expeller pressed canola is a fantastic choice.