In fact the Wayland devs are the primary group of people maintaining what is left of X11.
The reality is that a lot of X11 design is simply a bad idea. Which is what one would naturally expect given its pioneering nature. By the mid 1990s it was already obsolete and didn't match up with how hardware worked nor how people were using computers.
What has allowed it to persist to this day in Linux is more along the lines of:
* we just that we had a really good open source implementation with a very dedicated developer base and some commercial support from companies like Redhat.
* compatibility with X Servers other then XFree86/Xorg was no longer relevant since the rest of the world has long abandoned X11. So aside from a few nitch cases were people wanted X11 apps on their Windows desktops using properietary X Servers it no longer really mattered.
* X11 allowed extensions (most of which didn't work with network transparency, btw)
* Widget authors, mostly QT and GTK, went through great efforts to avoid using X11 and working around limitations.
KiCAD is just a unfortunate case. They use a kinda odd toolkit, wxWidgets, and happen to depend on a lot of X11 specific behavior for some of their application's features.
So it is less of a situation of Wayland devs not understanding X (they probably are some of the few people that actually understand it deeply).
Wayland actually has really good X11 comptatibility through XWayland and for most applications it doesn't cause issues. But for KiCAD it does. And they are just being honest about it.
Accomidations are being made for KiCAD type behavior and probably KiCAD will update and change things to meet them somewhere in the middle and in not too long it'll become a forgotten issue.