Some are, yes, this is incontrovertibly true. But this one? You should hit the "history" button that's in the corner of every wiki. That page hasn't been edited at all since 2013, imported from some other document written I don't know when, and literally the only thing it actually says about security is that "needs to be designed with security in mind".
What does that mean? What specific shortcomings have they identified?
By design Xorg has no isolation between clients so they can all read each others input, control others windows, and inject keystrokes into other applications. That’s unacceptable in the modern age and makes any attempt at sandboxing or separation of privileges for GUI applications completely pointless.
This "hole" doesn't exist. For an X client to capture input, it must be authenticated by either the unix user permission or by an access control list (where the default is to deny). Individual clients can also be marked untrusted which sandboxes them to some extent (though not as much as using a separate X server of course).
I'll grant that in practice, most the time these restrictions are very lax... in part because they can break some applications. But at the same time, in practice, it doesn't seem to matter that much since either you're running things you trust anyway or if a malicious application has access to your X connection they also have access to all your other files so you're in trouble anyway.