Great article! thanks :-)
People with a lot more knowledge of lens design can correct me if I am wrong, but I believe it is down to the lens being designed for a sensor as opposed to film, where the light needs to hit the sensor at a much straighter angle so as to minimise vignetting. I remember Leica using a sensor design with slightly offset sensels so people could still use their old lenses with minimal loss of performance.
I would also guess that this lens would perform much better wide open than an FD, but of course, we will have to wait until they have been compared in some capacity.