Yeah, that comment is a bit outdated/misleading.

By "pure python" it more means that there isn't any special support for e.g. putting a cc_library in it and having something magic happen that gives you an e.g. manylinux compatible output with the appropriate ABI tags in the wheel name.

A C++ shared library or CPython C extension shared library can still be packaged by it, it just has to be treated as an opaque data dependency. It's on the user to add a target that creates the desired output.