I'm not sure to understand the question. If a user has already used the "Sign in with SimpleLogin" on CollAnon, they will be automatically logged in the next time. There's a demo on https://forum.simplelogin.io
I tried it out in the end but didn't find "anonymous social login", which I assumed its gonna be a middle man service that supports the usual social logins like Facebook, Google, Instagram, etc., but it doesn't seem the case.