The Fediverse is federated on exactly the same layer as email. You have a username of the form user@domain,this is the same in both Mastodon and email.
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?
The fediverse has conversation streams that are mostly in public indexes, which other people can observe and interact with. In addition to these conversations comprised of statuses representing activities, it is also possible to follow the authors providing said content so that their latest posts show up in your stream, regardless of whether those posts pertained to that conversation or not.
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.
Sure, but when I said "in a relevant way" I was talking about differences that are relevant in the context of the conversation.
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.
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?