[dkersten]

This doesn’t seem like good advice to me, as others have already said.

C++ standard library is written in a heavily templates style most user code isn’t written in. That would be a terrible way to learn.

Some of Python’s standard library delegates to C code.

Much of Clojures standard library builds the language up from a small set of primitive, sometimes out of macros, and isn’ta good example of how to write good code yourself.

The standard libraries tend to be written in a different mindset than idiomatic user code is, as the requirements differ. In my opinion, the best way to learn a language is to implement a non trivial but small project in it.