This comes back to what federation and “the fediverse” is, and why trying to hide its nature is harming it.
No one expects their Facebook post history to follow them to Reddit, or to a forum, or to Lemmy, because they’re different websites. Just as no one expected their Twitter history to come with them to Mastodon.
But because it’s framed as “Mastodon” and not “social.website.com” the expectations are different.