I'm not sure what is going on here, but in a way it almost sounds like they are calling server side decorations legacy.
From the article it says "As of today, Mutter will style legacy titlebars (i.e. of X11 / Xwayland apps that don’t use client-side decorations) using Adwaita on GNOME."
If I'm reading this correctly they will style X11 / XWayland apps that depend on server side decorations? What about wayland apps that depend on server side decorations? Will those still just be broken windows that you need to know about the "meta + left click" shortcut to move?
Looking Glass is apparently broken on GNOME + Wayland in this exact way.
Ideally there would be a bare minimum server side decoration for Wayland apps (like Looking Glass) that don't provide any CSD. Hopefully that's on the horizon if it's not what's being discussed here.