for it to work as the opposite of a TC wouldn't it need to make the lens closer to the sensor? I suspect there is some optics in play as well as electronics.
Not sure if sticking a (albeit expensive) adapter with optics will ever improve picture clarity - it would increase FoV in the same way that a wide angle adapter on the ront of your lens would - dubious about the increase in exposure levels.
Aperture is the ratio of lens diameter to focal length - this adapter changes the field of view not focal length (that's determined by laws of physics) - clearly the diameter won' change and the focal length must increase by definition.
The only other way they could do it is by putting in a tiny black hole so that light is drawn into the lens - would cause quite bad distortion though I imagine.....