I'm talking about very basic background knowledge of general software engineering. Yes, a professional frontend developer should have at least an elementary understanding of the differences between the major programming languages.

An expert frontend developer will necessarily be competent in many languages, and might be competent to, for instance, contribute code to the Qt project.

I can see the value in the Project Management section of the article, as that's the sort of 'soft' topic that will be new to, say, a fresh graduate. If someone doesn't know the first thing about programming languages, though, they're a long way from the standard of a developer.