You got me thinking on this one. My rear bumper was recently replaced and the lead time for the part was well-beyond norm. The reason which BMW gave me for not having the part in stock in the US was the low number of MSport equipped diesels on the road. This could have been nonsense, but this may be a mark against your friends theory.
In looking at the parts, the part number for the diesel is 51128029687 and that for the petrol seems to be 51128041139. As the cover for the urea fill is listed separately, this would again lead me to believe that the are, in fact, different parts. That said, neither of these is a definitive answer, but both are good rebuttals to your friend's mockery.
...curious...