It is a matter of responsibility. If you can log into any lemmy instance or mastodon server with the same account, then which server takes responsibility for your actions in the fediverse?
I have seen instances be defederate from because of their lax account creation requirements, or because of harrasment from users from a specific instance.
If an account can log into any instance, then who is responsible for banning the account?
The RHEL 7 book from OP is most certainly still relevant. For example, my department at work has not managed to switch over to the brand new RHEL 8 machines just yet.