Gold is too soft to be able to write with directly. Were the hard tipping absent, a gold nib would wear down comparatively quickly. For this reason the iridium tip is applied to the point of the nib after electroplating. It forms the surface that actually runs over the paper and protects the soft nib blade from wear.
The nib slit is cut only afterwards, and indeed with cutting discs as thin as a hair, which carry the slit as far as the breather hole. The slit is decisive for the ink flow, since it is by way of the slit that the ink passes from the feed to the tip. The breather hole is punched earlier, together with the stamping of the chased decoration, and forms the end of the slit.