Authelia is an authentification provider. So you can have a single login for all your services. It can provide autorisation and authentification with a single unified login.
Bitwarden is much “simpler”, in it’s just a passwordmanager. As soon as you start sharing passwords, like you do in bitwarden, you lose the authentification part, even worse, you lose control over the shared login. Anyone with autorisation can “steal” the login as in unauthorized copying/distributing the password or even changing the password alltogether.
With an sso like authelia you can mitigate such attack vectors.