Only one man can control the flame, but Sarada is no man. And her sharingan was active when he said that. Amaterasu seems to be a blood line jutsu, not a kkg, and it directly connected to fugaku's bloodline. Kaguya said that Indra had it before. And It wouldn't surprise me if Sarada unlocks new ability in her one tomoe. Didn't obito unlock his intangibility prior his MS?