For info and to help anyone else confused by this.

As suspected when you create a new post to your own timeline feed in the “Visible to” field if you select Public this DOES NOT mean every site member (as is the site public) this mean to any Followers (or Friends who are automatically followers) Of your profile.

Note in some other forms when you can select Public (e.g. create Posts or Event forms). It means site-wide members.