Probably so others don’t experience the problems you had, yeah. But…
I just noticed that the find_package() call does not use REQUIRED, and you said above that KWin builds fine without the xwayland binary (a.k.a. it passes the KWIN_BUILD_X11=ON check).
A bug report to KWin detailing the specific issues you’ve had without XWayland makes sense since apparently XWayland is not a hard requirement for KWin like I previously thought.