No, it is not a sin to wear pants, and the article linked does not say that.
The topic is the differentiation in clothing between men and women and how blurring those distinctions can cause problems. Cardinal Siri is writing from a Western European standpoint, where pants were traditionally worn by men and dresses/skirts by women.
I think this article is almost prophetic, and it was my primary impetus towards dressing almost exclusively in dresses/skirts.