I share the author's opinion that xlib is harder to grok than the X11 protocol, because it's the X11 protocol plus a queuing system where you will sometimes receive out-of-band messages, plus a large number of seldom-used utilities.
(Talking about native implementations here not xlib bindings).
I remember seeing this on HN a few months ago and thought it was super cool.
Also just wanted to let you know that I sent you an email. Don't feel obligated to respond or anything, just wanted to let you know because I sometimes get emails from people on HN but they go to an inbox I don't often check so I often don't see them until long after unless they tell me in a comment that they emailed me.