Best I can figure out, it is possible to make a alkyd that has been spiked with another, tougher, resin - most often (for car and tractor paint) acrylic, but urethane can also be used. This seems to create a hybrid which falls somewhere between the tractor and utility alkyds and the expensive two part automotive finishes in performance. Better resistance to fading is one of the advantages often mentioned.
This would seem to be a useful thing, then, for those of us who are not trying to do a show finish, but rather just get some paint on a working tractor, to keep it from rusting away. But how good they are probably depends on how much urethane they are using - like the pancake syrup blends you buy in the supermarket that have 0.5% real maple syrup!