True -- trans fat is 5-10% of beef fat [1] BUT "a lot" is not the standard when it comes to trans fat. The NEJM article makes the point that trans fat is damaging even in small quantities, like a single gram. It was a great day when trans fat was required to be listed on the Nutrition Facts Label. It spurred snack manufacturers to get it (mostly) out of their products.
Natural tallow doesn’t contain a lot of trans fat. The only way to consume trans fats in large quantities is through hydrogenated fats.