My belief is that the ECU is reading voltage and another electrical measure (I don't know resistance or something) raw and doing the conversion to AFR internally. The sensor is quasi wide band in that it has signals like a wide band that need to be converted but doesn't have the accuracy range of a true wide band (I think because of the pressure from being pre-turbo. The front sensor is not a linear type like the rear narrow band so the ECU would not understand a linear signal at a 1v range or 5v range. Hence no aftermarket sensors can drive the close loop fuel control since the ECU also does not understand true wide band.
This is my understanding (which is not free from fault).