Plenty of people will still talk about providers that way, but it has never been about topology the way you describe. BGP is a mesh, not a tree. Packets that do have to leave one network are routed to the closest BGP peer. Plenty of small providers peer directly with other small providers (and always have).

It's also quite misleading to talk about the hostname once you've mentioned BGP. At that layer, the routers have completely "forgotten" the hostname, and are only looking at IP address/prefix to move that packet.

I understand why one might want to illustrate the fact that mail delivered via the SMTP protocol might be retrieved from the same server via the HTTP protocol running in different ports. But this is not a feature of TCP. The term "TCP multiplexing" commonly refers to reusing the same end-to-end TCP session for multiple application requests (e.g. more than one HTTP GET to the same server/port). Very different than IP multiplexing.

But email is also a very bad example to use to illustrate IP multiplexing, because email uses a store-and-forward mechanism very unlike HTTP lookups. A sender's email client just doesn't do lookups for a remote email provider. A mail client has a provider-specified SMTP server for all outbound mail traffic, regardless of destination. The client does use DNS to find that server's IP address. But the sender's provider's outbound email server performs an entirely different category of DNS lookup to figure out the destination mail server for the domain. This difference makes it a particularly bad idea to mention SMTP at all, only to say that it's unimportant.

I'm sorry this is such harsh criticism. It is extremely difficult to learn this material, let alone teach it to someone else in a digestible fashion. So please don't let this discourage you from trying to write a document like this. But it would be a lot more effective if you just didn't try at all to explain quite so much material in one post. Hope that helps a little?