If that’s the case how come it’s 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 swordsman’s overall strength and haki release
Like pre timeskip zoro with haki learned to cut iron but still couldn’t cut steel. Through growing stronger during the timeskip he can casually cut steel now but probably couldn’t cut diamond. Assuming he can’t now after learning to cut kaido he’ll eventually grow strong enough to cut diamond too