> In my view (and that of the documentation for sys.path), sys.path is where you put things that the Python interpreter can load - .so files, .pyc files and .py files.

It's quite typical for packages to install stuff in site-packages which the interpreter cannot load. In fact, that's the exactly the point of the package_data argument to setup(), see https://packaging.python.org/guides/distributing-packages-using-setuptools/#package-data

Just as an example, Numpy installs tons of stuff inside site-packages/numpy/ (header files, configuration files, data files for the testsuite)