Does this interoperate with Session (and perhaps other forks)?
No. This fork still uses Signal's servers. It's essentially an alternative Android client for Signal.
Session is a much more complicated protocol with way too many additions and differing specs for it to be able to interoperate with Signal at this point.
I see. I think it might be reasonable to clarify on the home page that only the client avoids google and blobs; the server uses whatever code signal.org chooses to deploy.
I agree. Also, the github repo calls it a "hardened" fork - listing some changes that they say improve security - but the website makes no mention of that.