You and the author are conflating two different kinds of "openness". The user is talking about the openness of the default visibility where you're presumably referring to the openness of the implementation.
In fairness, GP may also be envisaging a public (or intranet) 'mailing list archive' which is 'open' in the former sense.
This is exactly right. It is definitely possible that the email protocol could be as open as what I'm describing, but that has not functionally been my experience anywhere.