> Mean that /any/ published estimate is wrong.
You're overthinking the issue.
At any moment in time, there is a well-defined list of people who receive the emails posted to the list; the listserver software, mailman, has to keep one in order to deliver the messages. The list of subscribers is usually available on request from the email interface, but Knut Hofland, the admin for corpora, has disabled the function.
Of course those messages can be forwarded to other people, reposted to a blog or accessed through the web interface (and, conversely, they can go unread by the people to whom they are delivered). As you say, this makes the list of subscribers an imperfect proxy for actual readership.
Thing is, the question wasn't about readership, but about the number of subscribers.