I would say your historical reference is flawed. One could just as well have written an encrypted text on a sheet of paper that would be non-trivial to decrypt

I would say your historical reference is flawed. One could just as well have written an encrypted text on a sheet of paper that would be non-trivial to decrypt (e.g. Z-340). The difference, I think, is that it used to be far more effort to create the encrypted text than it is today.