I don't know the exact details of their setup but I would imagine if they have phones on the ship there's a network they can connect to on the ship that's not their starlink internet.
Aside from being able to possibly identify the starlink waveforms with passive RF surveillance or being able to identify the location of the ship through hacking spacex or their satellites, if they went back and forth between being connected on their phones to the ship network and the internet, their phones could have been compromised, leaving the possibility also of them being a perfect pivot point for hackers interested in exfilling important government secrets.
Overall just very bad opsec for a ship and definitely not a good idea.
Do we know these are civilian phones? My assumption was these are not civilian phones because why let them bring their phones if there's no cell network to operate on and no internet.
Edit: You might be right it mentions they can get Internet when it's not underway so maybe they have their civilian phones. I am not in the navy so I don't know the procedures. Still bad cuz of the other reasons plus some about giving them the ability to target those networks from a deauth'd perspective but yeah the last reason might not be the case.
It wouldn't shock me tho if they still had access to some like nonclassified but controlled info too on their phones.