Whenever I tell people that Mastodon isn't the Fediverse, the Mastodon stans start saying, "Oh, you sound like a guy who's complains that Linux isn't called GNU/Linux."
No, there's a massive difference.
To most people, whether you call something Linux or GNU/Linux doesn't affect the user experience of using the OS.
Referring to the Fediverse as "Mastodon" definitely affects the user experience of using Mastodon.