I agree. A farm is a type of factory, and thus has fairly controlled conditions compared to a public thoroughfare. Any number of solutions could work for a farm that would be untenable for self driving cars, such as marking the expected area with sensors to define the working area, outlining the area on a GPS map, running the equipment over the area one or more times to teach it, etc. None of these work as a general purpose solution to a self driving car, and they might not even be progenitors to the self driving technology we end up with.

A solution to a highly constrained version of a problem may not be applicable to the general purpose problem in any way.

Edit: mean to say "may not be applicable" in last sentence.