Dave,
using a bipolar transistors reversed BE diode, or n-JFETs to protect sensitive amplifier inputs, with ultra-low leakage currents ~pA, has been very common in metrology circuits.
Fluke and also Keithley often use that, maybe hp also, but I could not tell, who was the earliest adopter.
Flukes 845A null detector/µV-amplifier is from 1965, but still uses these ultra low leakage diodes, like CD12599, which all cost a fortune.
I think, I've seen that transistor solution already in early 1970ties circuits.
What's missing in your nice video, is a measurement on this leakage current at higher voltages, and a measurement on the zener voltage of the transistors.
In the 121GW, on its 50MOhm and 5 MOhm ranges, it's important to keep these currents really low, in the low pA range.
If high break-down voltage transistors are used, there may be higher leakage currents than on a 'normal' 5..7V break down-type.
I've already investigated on that 121GW problem a few months back:
https://www.eevblog.com/forum/testgear/eevblog-121gw-multimeter-issues/msg1770224/#msg1770224, and my final solution, using a jelly-bean n-JFET, which D-G or D_S diode withstands up to 30V, has a few pA leakage only:
https://www.eevblog.com/forum/testgear/eevblog-121gw-multimeter-issues/msg1790081/#msg1790081So this protection, either using n-JFETs, or (leakage - selected) bipolar transistors, will cure the 5M / 50M problem, but requires a re-calibration of the 121GW, as the temperature dependent leakage currents
of the former 1N4001s had been included in the former calibration.
Frank
PS: And yes, I'm glad, that UEI did NOT use TVS in that place!
PPS: Does this circuit now really protect the input of U9?
It looks like it replaces D8 only, that means that R_RLD is clamped at an (unspecified) BE-zener voltage of the transistor of maybe +/-25V, or more.
If IC U9 is operated at either 4V or 15V, that might damage its input pin15. And this voltage should not go below -0.6V.
I think, that circuit is a complete fail in this application.