I wonder how much "pad" they have built into their departure time.
Our cruise didn't leave until 6 pm because we waited for passengers whose flights were late. We sat on the verandah and watched 5 Disney buses (none with more than 4 or 5 people on board) fly into the cruise terminal, one literally screeching it's tires. We could see these folks running through the cruise terminal, with Disney staff rushing carts with their luggage onto the ship. The main gangway had been retracted, they boarded through the service ramps, which I think are on deck 2.
Rumor on the ship afterwards were that these folks (about 30) all had booked their airfare, transportation and trip insurance through Disney, and there was no way Disney was going to refund all that money if they could avoid it. It was an Eastern Cruise, so they could not have caught up with the ship until Tuesday in St. Maarteen.