GDPR Article 15 says that you (as data subject) have the right to obtain access to personal data being processed by the controller. This can certainly apply to

GDPR Article 15 says that you (as data subject) have the right to obtain access to personal data being processed by the controller. This can certainly apply to certain types of messages about you but is also limited to the personal data about you. In other words the controller can and usually even must redact the messages. The usual GDPR limitation apply, especially the request can be denied if it is unfounded or excessive. So, while in practice you most likely would not get anything useful out of such a request, I don't think the quote from the original article is as far fetched as you and sibling comments make it sound.