You can’t login to other instances because you don’t need to. And also they don’t have your login credentials (what a mess that would be). But all the content from that instance is already available to you on your homepage and you can comment on it, up vote it… Why would you want to?
I think they mean account-scale points. As in the 30 points you get from this post and 20 from a different post total to 50 karma. Pretty core feature on Reddit and doesn’t exist on Lemmy.