In 1993, Premier Cruise Lines operated the Big Red Boat. They had a contract with Disney (
DCL didn't exist then) to have Disney characters on their ships. Disney did not renew that contract when it expired...there were many reasons rumored--the company decided to quit loaning out their characters to anyone, they were not satisfied with the quality of Premier cruises, DCL was in the planning stages.
So, there was no way that Premier cruises (which is now bankrupt) was "indirectly" a Disney ship. Only a cruise on DCL qualifies you for the
Castaway club. That said, a Castaway club member does not get a discount on future cruises unless they are booked while on board. You do get a small gift--currently a beach towel and 2 pins. You are also invited to a reception with free drinks and eats.
If you have cruised on DCL and not received your Castaway Club card, you should call them.