What I can't find on the website is the case where a legit user that does already have an account on one of those socials and can still use that same account through this service.
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.