I don't think you can apply it to the stuff you booked before the promotion began, though.
Looking at the promo terms, it mentions:
> This offer is eligible for bookings on an eligible voyage purchased between 12:01 a.m. EDT on 01 May 2023 through 11:59 p.m. EST on 31 October 2023 for existing Sea Rovers, Sea Blazers, and 485c members.
> Eligible Voyages. Eligible voyage dates are between 12:01 a.m. EDT on 01 May 2023 through 11:59 p.m. EDT on 31 May 2024.
So, if you booked after 01 May 2023 and the voyage is in the eligible dates, they should be able to apply it, yeah.