But that still just creates a new Pixelfed account which trusts the Mastodon account identity, and it must be followed separatel from the Mastodon account. And you can’t enable the SSO between existing accounts, and there’s no account migration on Pixelfed.
A lot of people dont use an app at all, much less third party apps, so the whole thing was lost on them to begin with, if they were even aware of it.