This is not a very strong argument. Here's a specific refutation: the credentials to their primary email account are likely equivalent to the credentials of many other services that they use, because of password reset. None of those emails are encrypted, or ever will be; further, they're of little value just a day or two after they're sent. That commenter could coherently expect both that their mail spool would eventually be "public" and that it was safe to use email for password resets.

More generally: it's reasonable both to expect that your mail spool could eventually be public, and still not to want people to read it. There are things I don't want people to read, and there are things I need to be as careful as I can to ensure everyone can't read. Email works for the former and not the latter, and the latter is what encrypted messaging was invented for. Comparatively: I don't know many people who trust Twitter DMs, and "let's move this off Twitter DMs" is a constant refrain. But my answer to "can I read all your Twitter DMs" is still "no".