The Zeners already have a Zz = 9Ω @58mA  and Zzt = 400 Ω @ 1mA or the threshold  value.    

 The sum for Vf=0.7V and Vz=4.3 add up to 5.0V yielding a convenient regulated +/-5V clipped signal.

The Vz tolerance is +/-5% at 58mA but would rise 9mV/mA with more current and drop by 400mV/mA at Vzt. THus for stable operation using more current towards the rated current is preferred.

This ideal source impedance is ΔV/ΔIz= 10V/58mA = 172 Ohms which will not be affected by the AC input signal of +/-25mV over +/-5V output swing.

**My answer is 172 Ohms for driver source** as the Op Amp with high negative feedback has a Zout near zero.