Sorry, I couldn't get past the first couple paragraphs trying to tell me that the entirely open, federated-for-50-years, global standard is the closed system.
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.
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.