AW have always been cautious when it comes to cambelt change intervals. Mainly because failure is catastrophic and in 99% of cases requires an engine rebuild.
The manufacturer’s service schedule for the Giulietta is, apart from anything else, a way to reduce costs for fleet buyers by extending the intervals, encouraging them to buy a bunch of Gs instead of Focuses for example. That might work on very high annual mileages, but a 21k oil change interval on a domestic user’s car is crazy because of the likelihood of contamination from many short cold trips. Oil degrades much more quickly in those circumstances.
So AW’s advice is cautious but not overly so and much better than the cost of potential failure, particularly on MA engines which seem to be very sensitive to oil grade and quality.