You can send messages to anyone regardless of the instance they're on. This is also exactly the same on both Mastodon and email.
Precisely how are these different in any relevant way?
Email, conversely, often is a limited-scope conversation that can only be observed or interacted with between the people participating in said message. Of course, this changes slightly with the use-case of mailing lists, which often provide a public archive of prior messages. But email content is generally not accessible from the web in the same manner that a status is.
There was nothing wrong in your description of the differences between the Fediverse and email, but none of those differences explain the previous poster's assertion that the protocols work at fundamentally different levels.