It's the current cruise on the Fantasy that is having engine issues. She was supposed to go to
Castaway Cay today (9/8), but instead is skipping that and slowly going right back to Port Canaveral. They are being offered the discount (plus OBC) as compensation. AFAIK there are no changes to the Fantasy sailing that departs tomorrow (9/9).
I do agree with your take that since the discount offered was compensation for the inconvenience,
DCL is unlikely to offer that discount to someone who didn't experience that inconvenience because they weren't on the ship.