It's all about doping and junction area / depletion
volume. Small protection devices can stay sub-nA
if their doping is high enough to keep depletion
region volume low, so the generation current at
high temp is minimized. HV junctions encompass
more silicon volume within which e-h pairs that
are thermally generated, get swept to create
leakage current. A low voltage zener will be fairly
highly doped. It's likely to be much, much smaller
than the drain-body junction area as an ohm or
ten of clamp series resistance will suffice for HBM
ESD protection of a 20V gate oxide, while Rds(on)
is desired to be milliohms.
I've tested JFET-input op amps at 300C and seen
input leakage below 1uA, there. 1nA at 125C is not
out of the question for small signal devices. What
the clamp construction is, who knows?