It prob is a "some culture warrior/bodybuilder misread a piece of research and it turned into a culture war conspiracy" thing. Like soy. (My silly nickname predates the culture war stuff by several decades, it comes from Shadowrun).

E: yes looked it up, comes via RFK of course and is anti seed oils thing. https://www.diningandcooking.com/1742624/america-stopped-cooking-with-tallow-for-a-reason/ and the anti seeds oil thing is an idea that has been spreading through the (far) right blogosphere for a decade or more (that is when I heard of it, so prob existed before that).

Our friends of the pod at lw also joined in https://www.lesswrong.com/posts/DHkkL2GxhxoceLzua/thoughts-on-seed-oil (but it concludes the seed oil theory is full of shit, points for LW there, well done (dont read the comments, there is roundup in the seed oils)).