Comment by rcxdude

The current is measuring the rate of the reaction. With the two-terminal design the rate of the reaction is proportional to the rate of diffusion of the oxygen into the area where the reaction is taking place, which is related to the oxygen concentration around it but also can be affect by other things. With the third electrode, the current is proportional to the concentration of oxygen in the area around the sensor directly, which will equalise with its surroundings much more consistently than the rate of diffusion.

(A quick google brings up this document which describes the principle. No idea if this is the company in the story: https://semeatech.com/uploads/Tech_Docs/AN%20161205.pdf )