Yes, but as @lake54 wrote in Attachments available to any user with link those attachments are still accessible to people with the URL. So not foolproof in terms of limiting access to attachments.
FWIW, I am not especially enamored with forums that block access to content until you sign in and use teasers to get people to sign up. I guess I just don’t like to be teased. The private members-only goodies just never seem to be as good as I hope they would be.