While true, this phrasing may be a bit misleading. The concern at the time was that saturated fats were bad, and unsaturated fats were not as bad. But the unsaturated fats (think regular vegetable oil) is liquid at room temperature instead of a gel, and has different cooking characteristics.

But the process of hydrogenation turns that liquid (at room temp) oil into a form that is closer to the traditional animal fat oils (i.e., lard). It also happens that the hydrogenation produces as a byproduct a small amount of trans fat contamination (this was never the goal, to produce trans fats -- it is a hard to avoid side effect of hydrogenation). And at the time, the trans fats byproducts weren't thought to be as harmful as saturated fats, esp since they make up such a low percentage of the overall product. Of course, now science knows more, and corrections are being made.

Edit: Is my understanding above correct? I.e., is it the goal of hydrogenation to produce a more solid form of oil, and the trans fats are a byproduct? Or is it the trans fats that actually solidifies the oil?

They switched to vegetable oils, not to trans fats.