This can be done with Cauldron, an engine Castle has created, and currently only partners, moderators, and beta testers (who participated in the test with a link from Castle.xyz) can use it. Since anyone else who doesn't have permission to use the engine shouldn't use it, Castle automatically disables remixes and deck source for any deck with Cauldron. However, the deck you shared here has remixes and deck source enabled, so I don't know how they did it either.