It might help to think of it like email. If you sign up with gmail, you have a gmail account. You don’t necessarily have a microsoft account but people @outlook.com can still send your @gmail.com account email and you can still read it no matter where it’s from
In a similar way, you have a kbin.social account. You don’t have a lemmy account but you can still see posts from people in lemmy.world
animated emojis were never under server boosting as far as I know? I can’t think of a feature that Discord has made paid that was once free outside of trial periods… ever (with the possible exception of gamewisp emojis which I believe we should discount)