Okay, but liar appears to be targeting Harmattan too. Have a look at his YouTube link above.
Not necessarily. This may be true in some cases, but not for everything. Differences such as X11 and Wayland may need to be accounted for. In the case of namespaces, they are a feature of recent Linux kernels. systemd-nspawn uses the the same underlying kernel technology. Why waste time writing code twice when you can write it once and have it work on both?