Earlier quoted context omitted.
What on earth are you referring to?
That the security model on Unix (and Linux) is to trust your applications and mistrust other users of the same machine. While now the security model is that your applications are closed source and you cannot trust them, which is why you need wayland.
If that were true, httpd (and all other system daemons) would be run as root and neither the 'nobody' user and group nor the various security-related X11 extensions would exist.
Anyone who has worked in this field for more than a few years (regardless of their era of entry) knows that nontrivial programs are faulty and can happen to or be induced to do things that are harmful in varying degrees to the operation of the computer that runs them.