Perhaps this is a repeat thread, but this one is titled appropriately and in the Sony NEX forum, compared to the other.
As an old telescope user, this lens makes a lot of sense -- with an FF lens on a cropped sensor, you end up wasting a full stop's worth of image circle light that slops outside of the sensor area. This lens "concentrates" that light into a smaller image circle. Sort of the same concept as when you use a magnifying glass to light a fire by focusing sunlight into a smaller image circle. Wouldn't work with RF lenses because there isn't enough room for the optics, whereas SLR lenses have to leave room between the focal plane and the mount to accomodate the mirror box.
I've been putting off getting a NEX, but this is moving me a lot closer.