I guess the real story is that Ubuntu 18.04 LTS will use X.org by default, rather than Wayland [1]. Canonical "pretending Mir never existed" is just click bait. Personally, I found Unity not half-bad, and am not looking forward to gnome-shell from what little I know, eg. for its wasteful menu-per-window attachment rather than Mac OS style global menus alone.
[1]: https://insights.ubuntu.com/2018/01/26/bionic-beaver-18-04-l...