On the surface, it sounds good that only 19% of people in a very enclosed environment were infected.
But at the same time, consider this: Once 10 people had tested positive for the virus, they quarantined the entire ship, healthy and all. No one could leave their cabins except for a few minutes a day to walk around and get some fresh air before having to return to quarantine. Meals were brought to each room, instead of use of the dining areas.
In spite of those measures, from that initial 10 tests, another nearly 700 people were infected along the way.