Maybe this https://blog.kasson.com/a7riv/speculating-on-the-sony-a7riv-photographic-dynamic-range/ explains it, if they use downscaled 8mp images for DR comparison. Basically that implies the sensor is still the same with linear 14-bit ADC.I suppose that one could design a non linear ADC, but most designs are ladder style. Going non linear would be a lot more complex and would (presumably) be slower and take more power to run.
Besides, sensors are linear, and the number of bits required to sample the results depends on the full well capacity of the sensor. To gain an extra bit, you need twice the well depth, and Sony (or Canon) don’t have that.
Still I don't get how even downscaling can pump the DR above the theoretical limit of 14 stops.