I am also booked on this cruise and was told that indeed antigen tests were needed. I found this online from Costa Cruises parent company, the Carnival Corporation website:
“As of October 8th, on Costa cruises in the Mediterranean, the Caribbean and the United Arab Emirates, fully vaccinated guests will be able to embark without any further tests.
Costa Around-the-World, transatlantic cruises, and generally all cruises longer than 14 nights, require guests to be fully vaccinated and take an antigen test with a negative result within 48 hours of embarkation time.”