The first bullet point below that title is "First parties have a way to delegate access to a user identity to specific 3p's, as long as that delegated identity remains sharded by 1p." The model is proposing that if you allow Google to have access to user identities on your site, and I allow Google to have access to user identities on my site, the browser should prevent Google from joining those identities and detecting that the same user visited both of our sites.

(Disclosure: I work for Google, like the author of the article. Speaking only for myself.)