So to simplify this, from the sensors point of view it is like looking through a magnifying glass to get a greater fov?
The adapter is compressing the full image to fit on the sensor. (more or less but possibly not exactly)
And this:"five optical elements in four groups condenses the light from the full-frame lens and effectively reduces the crop factor"

​So it passes the image from my high IQ L lens through additional optical elements.
Most likely the IQ loss through the additional elements will off set any added benefit of the adapter.