is that the default value the graphql module is using?

I don't see any env variable declaring that path for the gunicorn service to work with.

(curious) Why would there not be one?

yes, it's hardcoded in the entrypoint of the docker image

A user may want to use an unsupported ingress controller (traefik or whatever) or may have no ingress controller at all in his cluster and for test purpose, just the service and kubectl proxy can be enough.