The problem here is that we're talking about Smith's LTV and not Marx's. Marx may provide his own reasons for believing in the LTV but he disproved Smith's. So the fact that Marx had these reasons are irrelevant if we're trying to justify Smith's adherence.
Though, it is worth pointing out Marx did develop his LTV in part by reading the likes of Smith and Ricardo. The reasons he gives for the LTV in Capital are absent in A Contribution to the Critique of Political Economy in which he merely takes LTV as a given. Actually, that's not quite right, he quotes Smith, Ricardo, and Franklin among others in support of LTV so it's my conjecture that he in fact did see an inconsistency and run with the theory anyway. It was only later in his career that he felt the need to justify that decision.