Yes. You're right, they claim detection is made with this "door knob" device, not with the camera. Of course, this is what they claim. No one knows what they actually do, nor whether there is anything active in this sensor, nor whether the app doesn't actually use the camera with a bogus algorithm while they claim using this external sensor. Or maybe the app just gives random results without using any sensor at all.
But as I mentioned, the real question to me is: how could this ever get approved. It boggles the mind, and is extremely concerning IMHO.