I think you are misunderstanding... They got dumps of a couple of databases, which they encrypted, downloaded, and presumably decrypted. Those dumps themselve

I think you are misunderstanding... They got dumps of a couple of databases, which they encrypted, downloaded, and presumably decrypted. Those dumps themselves contained a credit card number that was encrypted by Marriott with a key they don't think the attackers got. That is my read of it anyway.