I don’t see anything wrong with what you were told. It sounds like you both had a relatively mild case - symptom onset was absolutely horrible horrible timing, though you most likely “got Covid” before ever boarding - and did the responsible thing by self-quarantining. There’s really nothing NCL could have done for you more than you already did for yourself and they were completely transparent regarding the costs so they were trying to do the right thing by you in that sense.
If you really felt you needed to go to medical, you still could have. And you would have been triaged, waited forever then and billed heavily same as if you had an ER visit on land.