That there exist SELinux plugins does not mean that XACE itself depends on it. Like, try reading the actual documentation instead of regurgitating FUD: https://www.x.org/releases/X11R7.6/doc/xorg-docs/specs/Xserv...
It's also outside the scope of a desktop environment or window manager to ship plugins to the X server. Hence another reason why they had to go with a new protocol that makes it easier to implement their own server...
I stopped reading at this point. If you're not going to read the documentation, then you can kindly go crawl back under your rock on /r/linuxmasterrace.
Yes people could develop new plugins that integrate with some other security mechanism, but they haven't, in part because the hooks are so out of date, and in part because, you know, that requires building another security mechanism. The access hooks are not a security mechanism, they allow you to integrate with some external MAC.
How does X help here?