Over in the Unity visionOS forum, a Unity rep has said that their experimental package has only been compiled for Apple silicon and against Beta 2. It will take some time to shake out for Intel and more current Xcode beta releases. https://forum.unity.com/forums/visionos.968/

I believe I have figured out the issue. The QuickTime movie has to be named with _video at the end. Even if your URL path matches the name, if the name does not end with _video.mov it will not work in this particular example project.