I didn't say Marx invented the Division of Labor notion, but, as others have pointed out, he made extensive use of it and much of his thinking really hangs on the social differentiation which he said comes from the Division of Labor based on production.
Anyone claiming to understand economics must understand the Division of Labor. It is not a Marxist concept. Adam Smith demonstrated the critical role the DoL plays in a modern economy.
Nor does it play much of a role in Marxian economics. I have no idea where you got that idea. Marx's social differentiation essentially had three components: proletariat, workers and petit bourgeoise and those were determined by the relation to the means of production.