XCB is, per wikipedia, "comparable, but slightly lower-level"; read that with emphasis on "slightly". It's certainly better than xlib, and if you were to write a real WM from scratch these days you should probably use it, but I think bringing it up is sort of besides the point in this conversation: OP was wondering if any sort of cheating was going on by including an X protocol library, and the answer is "no": neither xlib nor XCB would be appreciably providing any sort of abstraction around the concept of "X window manager".
