This topic is related to different stuff from that. Accounts are exclusive to their instance, and it's the instance (e.g. lemmy.world) that talks to other ones (e.g. lemmy.ml).
Your lemmy.world account is only for lemmy.world and can't sign in to any other Lemmy instance, but it doesn't need to, because you can access all other Lemmy instances that Lemmy.world is linked to and hasn't blocked. So you can go to [email protected] from here by either searching that in the topbar, or you should be able to via this link: [email protected]
It can't be quite confusing but @[email protected] is right. You can subscribe to communities on other instances but you need to login to your own "home" instance and work from there. Might be helpful to have a look at this post for some insight into Lemmy.