that appears to be conflating protocol and network
remember when email was not all smtp and imap, and addresses had to specify the path a message was to take to reach the destination? when uucp and smtp and other protocols coexisted and allowed mediated interoperation? it was the email network, even while some used only smtp to interoperate
activitypub is analogous to smtp; the fediverse is analogous to the email network
same reasoning goes to ipv4 vs internet. ipv4 doesn't define the boundaries of the internet, nor do they overlap fully, though for an e.g. ipv6-challenged party it may seem like it.