This might be pedantic of me, but conflating RLS and auth isn't
a great look. RLS is a general purpose mechanism for constraining row operations, and auth has to do with usernames and passwords and session tokens.
We are still doing a heavy assessment of whether this model can be generalised for everyone. It covers the details of both authentication and authorization - we are just building a nice/easy way to enable this for everyone (probably using the same model as Postgraphile: https://www.graphile.org/postgraphile/security/)
Probably similar in an overall sense to Postgraphile et al[0], in case you haven't seen that - although I am also interested in the specifics relating to Supabase.