St. Paul doesn't seem to be talking about solely a priori here, but rational discovery of natural law.
I would agree that throughout history those without scripture developed a posteriori deductions from creation. But, also keep in mind that Israel was located in the middle of civilization (Golden Crescent) and because of trade a lot of a posteriori knowledge of the Jewish scripture was extended beyond Israel. Intially, however, we all are born with a conscience which is certainly a priori knowledge. It's the rejection of that a priori knowledge that leads to the worst sorts of evils.
God gave them over to a depraved mind, to do those things which are not proper.