But encoding is relevant here too. If people cannot make sense of the messages, whether or not they're cryptographically secure, the messages aren't fulfilling the pedagogical purpose of the amateur service.
But you can record the Hiatian speakers message and get it translated.
With sufficient encryption, recorded messages cannot ever be decoded unless the key was broadcast, which it never would be.
However, this exact form of encoding was used as a form of encryption while the US was at war; the Navajo Code Talkers.
Somewhat off topic, of course, but a great example of how encryption and encoding can have identical end results.