[boffinAudio]

I think where you're getting confused is that you're replicating local functionality that iTerm2 offers (cmd-D for a new terminal session) versus tmux features that are important and useful when used on a remote host.

Using tmux on a remote host means that you can always detach and re-attach to the remote host session when you want to. You can do that on your local host too, but this is only useful, ultimately, if you have long-running process that you want to leave running - and be able to re-attach to - while also being able to quite iTerm2 completely ..