Tableau Online requires a valid username/password to see anything. Therefore it's a lot more secure than Tableau Public.
Tableau Online is Tableau Server, just hosted and maintained in the cloud by Tableau.
On the pages for the product Tableau Online it's stated: Publish dashboards and share your discoveries with anyone.
If I were to share visualisations/vizs publicly for anyone to see (without logging in), is it correct to say that the only platform is Tableau Public? There is no other option?
Sharing with "anyone" with Tableau Online means they need to have a username & password
Sharing with "anyone" with Tableau Public means anyone on the internet can see it.