I would call it "better than nothing"-documentation as you probably have to be a bit desperate to look here for docs. Probably took effort to reverse engineer the behavior too or from someone that knows the in and outs of the mac window manager.

Although thinking about it, it probably is common for window managers to have some kind of drag map because they also need to to allow to drag stuff with apps that completely crashed.

But yes, really nice that the commit explained the change. By experience more extensive messages are available where others also invested an unexpected amount of time.