What credentials are handled in this section of git-auth-helper?

Asking about these credentials at line 53.  Conversion to base64 would be a concern if the credentials are provided by a user and represent an actual Github user account?

https://github.com/actions/checkout/blob/25a956c84d5dd820d28caab9f86b8d183aeeff3d/src/git-auth-helper.ts#L53