It doesn’t require any mental model more complicated than email to understand this. They’re running a server so people in their org can have mastodon addresses under @archive.org. You can follow, send, or receive messages to mastodon accounts on the archive.org domain, but you can’t have an account there yourself.

Similarly, you can send emails to people with @archive.org email addresses, but you can’t send email from an @archive.org email address (unless somebody at the internet archive let’s you)