Device A knows about device B as soon as A sees another public encryption key in a Nostr event signed by the same user. So A and B do *not* have a different Nostr keypair. This is about syncing encrypted data on different devices for the same user I believe.
Discussion
Right, I assumed that must be it. The title of the diagram, inferring that the diagram itself deals with identity keys, is probably why several people (incl. me) were confused?