Call back & try again, as this either just isn't right & they're both ignorant (which is possible as Disney's rules are often different from other cruise lines), or they just recently changed the policy, because I paid Costco with Disney gift cards last year. If you talk to a manager again who insists you can't, ask them
when the policy changed, because Costco used to allow that. If they say it's never been policy it means they're just clueless. I'm still assuming they're clueless as they're saying you can pay off a
Disney cruise with a Costco Shop Card, which is impossible as the money goes directly to Disney, not Costco.