input protection, anti parallel diodes.
True, but more than just that, the real point is that they are using the base-emitter junction of two transistors that are on the same die.
The base-emitter junction of many BJTs have just a few pico-amps of reverse leakage. This is very important here because you don't want the protection diodes to leak current, since the electrometer has such a high input impedance at 200T ohm. Any leakage currents in the input path will cause measurement errors, so they must be minimized and controlled.
So yes, these are anti-parallel input protection diodes, but the main reason for using a BJT here instead of an average diode is the lowest possible reverse leakage current . They possibly need to be on the same die to keep them matched for temperature (or maybe they selected them for lowest possible leakage as a pair).