@moduledoc false and @doc false are conventions for internally used functions that need to be exposed because they are used through the project, but are also not part of the public interface of the library.

The reason why you can’t call the function, is because the source code you looked at does probably not match the elixir version you have installed.

This is the reason why the module is marked “internal”, this gives the core team the flexibility to change it as necessary without needing to think about backwards compatibility.

Also do note that IO.ANSI.Docs is @moduledoc false which means it is an internal implementation detail of Elixir and you should not be calling it from your code.