although it is so frequently repeated by those thoroughly unfamiliar with Marx it seems to have reached the status of self-evident and self-reinforcing meme, one that punishes any who question it.

Marx and Smith did not arrive at different conclusions, in fact, Marx, respected Smith highly, and cited him frequently in his work.
Marx recognized Smith had correctly described HOW economies worked, he (Marx) tried to determine WHY they worked that way.

You don't have to be a Marxist to recognize the enormous contributions Marx made to our understanding of political economy, including the interactions between politics, economics, and history. Among scholars who do not feel the need to reflexively reject him for purely ideological reasons, even his strongest critics still give him credit for his many insights. I like to think of the relationship between Smith and Marx as that between Newton and Einstein. The latter built on and extended the former, it did not replace or refute it. Think of Marx as relativistic economics.

Where Marx got into trouble is when he got out of descriptive analysis and moved into predictive synthesis. Marx correctly determined why 19th century economic activity was so destructive of human values, and that it was inevitably so; what he failed to do was come up with a workable modification or alternative to it.

Demonstrating that workable modifications and alternatives were possible had to wait for the twentieth century.