As people have been saying a lot, I'm pretty sure Yensid would have told DG that there were many keyblade masters since he works with him, 10 years down the line, Mickey wants them to go retrieve one, since they knew there were past ones, they probably thought he had little to no relation to the ones they saw long ago (TAV). So why mention someone they think would mean nothing to Sora, they are on a journey to find their king, not spread light about what happened before Sora was potty-trained.