The first half of the sentence you quoted is:
Explore the possibility of a single account system that allows users to seamlessly log into multiple Fediverse platforms
So one account for a Lemmy instance, another for a Mastodon instance, another for a Friendica instance, another for Loops, another for PeerTube, etc.
A few ways to contribute to move these ideas forward:
If you know one of either Python or HTML+CSS then you can start contributing to PieFed (explanation, which is much easier than learning Rust (+ also JavaScript) for Lemmy.
There is just so much low-hanging fruit there - like in Lemmy why can I only see upvotes separately from downvotes when viewing the web UI from a mobile device but not a desktop one, and even then for posts those values are below the post, alongside the actual voting voting, whereas for comments that identical information is way up above the content in a whole separate area, sometimes on the right of the screen but other times to the left. Ofc there are apps but those fall behind too, as new changes come out.
Similarly, Sublinks uses Java, and Mastodon uses mostly Ruby on Rails and JavaScript (description).
Or Mbin already combines Lemmy + Mastodon, written in PHP (requirements).
Or make a community on an existing platform, and invite people to post and join as you do the work of moderation.
Or help with funding bc server hardware and networking isn’t free. Or start one of your own and invite people to join.
Lots of work is happening. Lots more remains to be done.