ESD Protection Query
The Zener turns on when the inputs rise (Vzener + Vdiode) above GND.

If 6volt zener (a typical number, for highly doped junctions) + 1volt Diode (at human-body currents of 1amp), as Vesd rises to 7 volts, the Zener comes into action.

The ESD energy has to be dissipated DEEP into the substrate; the ESD design task included steering the current flows into deep resistive regions and dumping that energy.