I've been able to call into the Nim side from Python by treating the Nim code as C functions to call. That works, but it would be interesting to come up with a more Pythonic way of doing that.
I saw the original link on my phone yesterday. The screenshots in the post were unreadable since pinch and zoom was disabled, so I didn't fully understand until pulling up HN on a larger screen today.
I have no idea how many other people might have read your submission on their phone and had the same issue though.
I haven't looked closely at the Vala stuff that you mentioned, but it is very straightforward to generate shared libraries in Nim with exported functions that you can access via ctypes. That's not quite the same as generating a python module that you can directly import, but it gets the job done.