The article missed another point: they're competing with US based tourism. Every person who chose to go on a cruise didn't visit a US city and spend money there.
On the Caribbean and Mexico cruises we've taken, we spent a few days before and after in the US ports Fort Lauderdale and San Diego, so it is common for both same-day flyers and short term stay visitors to embark.
Not really it means they spent time on a boat not a plane. If anything a boat to some nearby island beats flying to somewhere geographically remote because the money will be less likely to flow back via tourists visiting from the opposite direction.