It didn’t have a back door. A third party client was used whose only purpose was to log conversations. That third party client’s archive was then hacked. Any

It didn’t have a back door. A third party client was used whose only purpose was to log conversations. That third party client’s archive was then hacked. Any client must by definition have access to the unencrypted data (in order to display it to the user). If you deliberately create a client that logs all communication, well, then all communication will be logged.