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.

Also nobody really uses zeners for this due to the soft limiting characteristics using a Zener to change gain and limit the voltage .

Why is this a terrible design example?

• there are no specs for Voltage tolerance and squareness of peaks.

• Zeners are poor tolerance to begin with due to the exponential VI curve

• The Source impedance must be higher than Zeners to reduce the limit the gain and thus the voltage output, thus the current is low and the knee is very rounded

• the Zener voltage drops well below rated Vz by more than 10% so it will never be 4.3+0.7=5V , rather more like 4.4V with 1K source

• at this point you ought to be re-examining the real requirements and specs.

  Why do I need this?
  Why do we take for granted examples in books and websites are perfect? Trust but verify. Never assume unless you know for sure.