The repo cruise arrives at 7am so that isn't the reason. It is typical for RCI to state that boarding starts three hours before the scheduled departure time. For instance, cruises leaving from Miami at 4:30 say boarding commences at 1:30 in the docs. But guests are checking in as early as 11am.
Do your docs use the specific language you posted? "Despite the 8:30 departure, boarding commences at 5:30" Or do they simply say "boarding commences at 5:30"?
I'd say if you showed up earlier you would find check-in open for business. 1pm for instance, with the buffet lunch wide open.