Well Mihawk is in the same rank as Ace and Murakumogiri, why is said to be the strongest sword?
Among the sword grades there are ranks, among the 12 Yoru is ranked 1st since it’s the strongest blade.
It always shocks me when I hear people say Emma will be 1 of the 12 supreme blades when it was never said, all Hitetsu said was that once Zoro turns Emma into a black blade it’s rank will increase never did he say it will change its grade.
Like the 12 supreme blade the 21 great blades also should have ranks and if Zoro turns Emma black it should be ranks as the highest among the 21 it won’t magically become one of the 12 blades