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The law of comparative advantage is best understood as a priori in its logical form: "if agents differ in relative productivity, then mutual gains from specialization are possible" is analytically true given the concepts of opportunity cost and exchange. However, you're right to press on this: the antecedent condition of human heterogeneity is not derivable from the action axiom alone. It's an empirical fact about the world, not a logical necessity. So the law occupies a hybrid status: the inferential structure is a priori, but its real-world applicability and social significance depend on contingent facts. This is actually a broader tension within praxeology - the action axiom may be apodictic, but substantive economic laws typically require auxiliary assumptions (heterogeneity, multiple goods, time preference) that smuggle in empirical content.