Trans fats are a natural component of dairy and meat, particularly beef. Prior to the systematic removal of trans fats from processed foods (e.g. hydrogenated oil), trans fats from natural sources constituted 20% of dietary trans fats. See, e.g., https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3551118/ That's significant considering the prevalence of foodstuffs like margarine and Crisco back then. Today they likely constitute a greater percentage of dietary trans fats. And while total trans fats intake may be declining, if people substitute hydrogenated oil with these "natural" fats, the societal health benefits will be seriously blunted.

This is complete, unsubstantiated conjecture on my part, but perhaps the evidence that showed that dairy and red meat were relatively bad for cardiovascular health had something to do with their comparatively large percentage of trans fats.