There's a difference between learning about programming languages and learning about programming language theory. The class you linked is squarely in the former

There's a difference between learning about programming languages and learning about programming language theory. The class you linked is squarely in the former camp. It's a bit confusing because they use PLT to stand for "Programming Languages and Translators" where I usually see it referring to "Programming Language Theory". Honestly, "programming language theory" is a bit of a misnomer. It's less a theory for programming languages and more a theory of CS from a language perspective. (Where theoretical CS is a theory of CS from a computational point of view.) Programming language theory is interesting in and of itself and is fairly distinct from the sorts of things you'd learn in a normal programming languages course or by implementing your own language. I'm not saying either of those is useless—I'm a big fan of doing both!—just that they accomplish fundamentally different things. Your advice is good if you just want to implement a programming language; the linked repository is good if you want to explore PL theory. PL theory shares a lot with formal logic and the foundations of mathematics. The main areas are things like type theory (a special approach to formal logic) and semantics (akin to proof theory or model theory). What you'll learn by reading the books linked above is almost completely disjoint from what the class you linked covers.