The author most probably uses the word "startling" in reference to the Law of Comparative Advantage because:
it is puzzling that no one before Adam Smith thought of the Law
the Law of Comparative Advantage holds even when there is an imbalance in the capabilities of the nations
it is surprising that the general public is unaware of the Law of Comparative Advantage
most countries do not consider the Law of Comparative Advantage when devising their trade policies