If thatโs the case how come itโs differentiated in moments like these
While armament haki (specifically emissions) is probably necessary for any swordsman to be able to cut iron, steel, diamond, etc. it likely depends on the swordsmanโs overall strength and haki release
Like pre timeskip zoro with haki learned to cut iron but still couldnโt cut steel. Through growing stronger during the timeskip he can casually cut steel now but probably couldnโt cut diamond. Assuming he canโt now after learning to cut kaido heโll eventually grow strong enough to cut diamond too