The mental model that made tmux click: tmux the command is a client. The
first invocation starts a long-lived server process, and every pane in every
session is a pty owned by that server, not by your shell.
This is why detaching loses nothing. Your client dies, the server keeps the ptys open, the processes inside never learn anything happened.
It also explains the failure mode where a stale server holds an old environment: you are not restarting a wrapper, you are talking to a daemon that outlived the thing that configured it.
See also tmux-snippets.