I think it reffered to Itachi's Susanoo. Itachi needed both Tsukyomi and Amaterasu to unlock his Susanoo. As in, he needed both of his eyes' powers to unlock it.
I thought the same thing until every Uchiha who has awakened MS showed their own unique abilities. Shisui, Koto Amatsukami. Obito's, Kamui. Itachi, Amaterasu and Tsukyomi. Sasuke, Amaterasu and Blaze Release or Flame Control (whichever is it).
You need to unlock the two MS abilities to awaken Susanoo. Itachi and Sasuke awakened their two unique abilities so they awakened Susanoo (even then, it was said to be a rarity or perhaps that's what it meant since not all awakened their two MS abilities). Though Sasuke and Itachi both have Amaterasu, it could be because they are related or because Itachi placed Amaterasu in his eye before he died. Madara has Susanoo so it's also safe to say that he has his own unique MS abilities. Otherwise, Itachi and Sasuke both would be able to use Koto Amatsukami and Kamui on their own as well.
This is my opinion.