As most of the underlying work is in QtWayland, I’d say: yes, it’s Wayland only (not being the author nor an expert on display technology, so take my words with some caution). The blog post talks about some stuff that is impossible to do in X11 (“impossible” as in “would require massive changes to X11 itself, which no one wants to do”).
@Sina@MartinR xorg supported exchanging the compositor before (kwin --replace). I believe xorg didn't support clients surviving a crash of xorg itself.