False, xorg isn’t written with support for Nvidia, when xwayland windows flickers on Nvidia it’s an effect of xorg not working Nvidia.
The Nvidia driver is a closed source implementation of the xorg server written by Nvidia for Nvidia GPUs. Xorg was invented at a time when drivers were done like that.
Now xorg uses glamor (except on Nvidia) which is a driver that implements the server over opengl, so you don’t need to implement the whole thing for every GPU. Except glamor doesn’t work on Nvidia because Nvidia doesn’t implement implicit sync, which is required by Linux, and that is what you see in xwayland (which uses glamor as well).
Wayland doesn’t require writing a whole server, but it requires implementing GBM and implicit sync (as does everything on Linux, unless you are using Nvidia’s proprietary corgi server). Nvidia refused GBM until a few years ago, and still refuses to implement implicit sync. Which is why explicit sync will solve most issues.