Lemmy doesn't support LDAP, OIDC, or anything like them yet, so at least as far as that goes sign-in can't be easily unified.
There are tracking issues though:
https://github.com/LemmyNet/lemmy/issues/489
https://github.com/LemmyNet/lemmy/issues/1241
https://github.com/LemmyNet/lemmy/issues/1368
https://github.com/LemmyNet/lemmy/issues/2930