As a person who was born a catholic I can only say that the pope is a bit late. The founders of our republic reached that conclusion in 1776..
With respect, the American founders got it from Thomas Aquinas and the entire Christian tradition. the Pope is not teaching something new, just reaffirming the 2000 year teaching of the Church.
Natural law theory was around long before 1776! The Founders were well aware of it, and its political implications had been discussed by people like the 16th century Spanish Jesuit Francisco Suarez and were quite well known in 18th century thought.