One way to prevent this is would be to re-sign the app with your own signing key and delete that key before court, I guess. But those people whose conversations appeared probably just had Google Drive plaintext backups enabled.

You can easily access any undeleted convo in any app if you achieve device access. I would like to read more about this to understand it more and because your reply is still a little unprecise, do you have links to examples?

Source for this