It was allowed back in the day. SMF had this feature to enable/disable guest posts, not sure if the feature is still there or not. However, although it seems like it was also allowed in Bitcointalk to post as a guest, sometimes, I get confused because, in some of the very old threads, some anonymous were changed into anonymous. They had usernames too. I can't remember if I had read regarding this prior to this post but as dar as I can remember, it was also allowed in Bitcointalk.