Yeah, it's not SSO, that's why I wrote "of sorts", but the post is precisely of how Pixelfed will now allow Mastodon users to log into a Pixelfed instance with their Mastodon account, meaning they will not see the current view of pixelfed as looked from a Mastodon interface but should look as if the user had a pixelfed account, since they are using the pixelfed interface.
It's not released yet, so I have not tested it, but hopefully soon.