I found one solution to the original problem of linking.

Expanding the docstring to include a sphinx-needs item, as seen in https://sphinx-needs-demo.readthedocs.io/en/latest/_modules/automotive_adas.html#AdaptiveCruiseControl provides a usable link using the :links: keyword

"""
.. impl:: Validate the user based on the given context
    :id: I_AUTH_1
    :links: R_AUTH_1

    More details about the implementation worth documenting
"""

Sadly it does not generate an incoming link at the original requirement at the moment, but this seems like another question since it is reproducable with jsut two rst files and independent of docstrings.