Public mailing list archive is common. An archive needs to be public to everyone because people need to search it even if they are not subscribers, or they may have subscribed before but decided to unsubscribe but still want to browse it to follow the list or to search it occasionally.
An email sent to a public mailing list will become a public record. Nabble is just one archiver, there are several other third-party archives and also there is the native archive by the mailing list server. We do our best to allow you to delete your own posts (most archives don't allow you to do this by yourself), but we cannot allow you to edit or delete other people's posts. So you did the right thing to ask the post owner for permission.