Simple, this is not an addon.
Arnold has no support for blender. Tyler says he’s working on it, he is active on the discord. But we see no progress on the development.
He claims to be working on an addon that makes it super easy, but there is no sign of anything. He refuses to show any kind of development. Only Screenshots of the render engine “Arnold” being selected, which can be faked within minutes.
Arnold will have to wait for another person to make an addon for it.
Tyler claims he is polishing the last 0.0001% (his actual words) of the plugin, but refuses to show anything. Which makes many belive he is just faking it for attantion.