I believe they could have also used the CBP ROAM app on their smartphone? They would have wanted to create an account, upload docs, etc, beforehand. It's my understanding you do not have to be a US citizen to use this feature.
When in US waters and with cell
service, request check in and wait. Either they will clear you in right away, or do a video chat with a CBP officer, or possibly be asked to report in person...
US citizens who are also "trusted travelers" report they do not have to report or do the interview ... click check in, wait, get clearance message back.