One thing that ARKit is able to do is utilize information about the device motion from the accelerometer and gyroscope. Also, I believe it is a slightly different technique than what is described in this post. I think they do more of a 3D reconstruction (i.e. structure from motion), and build up a sparse model of the environment, kind of like SLAM. That can perform better because it uses the knowledge that the environment is relatively constant between frames. This post, and most examples you see, essentially start over from scratch each frame, throwing away all the information from the last tracking frame.

Of course, many of the algorithms you need to do that are part of OpenCV, so maybe it is still 95% OpenCV!