I wish you were right, but I think you are mistaken. I copied this from the Carnival website and it says “up to date” is when one has all recommended boosters.
TESTING REQUIREMENTS
Except where destination requirements differ*, guests who are up to date with their vaccines may take their test within three days prior to sailing.
Up to date means a guest has received all recommended COVID-19 vaccinations, including any booster dose, when eligible.
Example: For a sailing departing on Saturday, guests may take their test as early as Wednesday and as late as Saturday morning, if guaranteed to receive results in time for check-in.
If a guest is fully vaccinated but not up to date with their vaccines, they must take their pre-cruise COVID-19 test within two days prior to sailing.
If you are eligible for a booster and have not received one, you are not considered up to date.
Example: For a sailing departing on Saturday, guests may take their test no earlier than Thursday and as late as Saturday morning, if guaranteed to receive results in time for check-in.