Async X11 was tried with xcb, and it failed. In the above linked video, this is illustrated in the "Bad IPC" part, with the example of gedit startup (gedit uses gtk and gtk uses xcb) and where it blocks.
Since this video is coming up on ten years old, it might have been true at the time, and gtk/gedit have since changed the implementation. But regardless, if the video is accurate, they didn't use the non-blocking calls. If the video is not accurate, it is meaningless anyway.