Hi
I was told by my local Fiat Professional dealer that 6 years should be OK for a low mileage-per-year vehicle. On the other hand, if the belt failed at just over 5 years I'm not sure they would be the ones picking up the bill for a new engine.
The handbook advice isn't ambiguous, it's 5 years or 180,000 km (112,000 miles) whichever comes first.