Seems to work here. I selected on the IT_IT website FCO-GRU on 17/09 and 24/09 and a RT fare of about 900€ is shown. At this point I inserted the coupon and clicked on "applica" and I got a discount of 180€, for a total of 718.38€.

Repeated the exercise for GRU-FCO on the same dates and the coupon is not valid. So I guess it only works ex-Italy.