@the_spc well, that depends on your own view of "public".
Unlisted posts don't go to the public timelines like the local timeline or federated timeline.
However, if you go to your profile, those unlisted posts are still publicly visible to anyone that visits it, on the web or any of the apps.
The only way to make your posts private are to set them to followers only. Even then, if one of your followers reports your posts, the admins could see them.