Interesting I've been downvoted for that when others have posted the same comment since and not been downvoted. Further more I'd with no reply it's unclear how the person who reviewed that comment believes you could secure the message after someone has read it (clearly they think it's possible otherwise my comment wouldn't be "incorrect").

To expand on my previous post: what's to stop the recipient from taking a photo of the message using another device? Or transcribing it and republishing it manually? Or even just memorising it and telling someone else verbally?

Literally the only defence you have against the recipient abusing their trust is not to send the message to begin with. Everything else is just security theatre.

This is literally the same point others have made too. So why I was downvoted but others not I don't know.

This. Or, in other words: OFFLINE COMMUNICATION.

In today's world, offline communication is safer, because to tap it you need manpower ( = more resources, more money ), and that is not efficient for those who want to tap you.