We need to ignore any reference to 12,000 miles for the 159/Brera as that is for the older cars.
Then if you look at the schedules on the maintenance programme it clearly states the intervals are only in multiples of 18,000 miles. Low mileage is less than 9,000 miles a year. It also adds
"All vehicles must be serviced in accordance with the full service every 2 years
Furthermore on JTDs, the oil and filter change is not
part of the service as "The engine oil and filter should be changed depending on condition, shown by means of a warning light or message in the instrument panel or every 2 years".
So my understanding - and that of my supplying dealer - is that you drive until your service indicator tells you you need a service - as you do with a 156. This is the case with most modern cars. However in typical Alfa UK fashion they have produced a contradictory service manual, not relayed the same information to all dealers and probably haven't got a clue themselves.
For what it's worth I'm servicing mine every 18,000 miles and had an early oil change at 9,000 and at part ex time if I get told it hasn't got a full service I will wipe the salesman's face with a rag dipped in Selenia.