You've confused me. Not hard to do, so don't feel bad.
My thinking, which may be flawed, is that this small diesel, maybe 2 cylinders would run at a constant rpm optimized for maximum fuel efficiency. This diesel would turn the generator, which would provide power to an electric motor or motors, to run the drive wheels. If a locomotive does this for torque, well, torque is a pretty good thing in a tractor. Plus, individual electric motors for each driven wheel would allow computerized traction control for less slippage (ideal for large acerage farming).
The principle of the small diesel is similar to the principle for APU's on large aircraft. Its a tiny jet turbine that provides power via generator for the aircraft when the main engines are off.
But, I suspect that in principle you are right and that if someone did the math, a diesel electric tractor might burn exactly the saem amount as a straight diesel. It just seems to me, that the diesel electric principle could be applied to one of these:
For the same reason it works in a locomotive. Presumably this is not the case or some smart fellow at John Deere would have already invented it. Oh well.