Could be, I don't know who makes it exactly. And while your thinking is not at all flawed with using what they recommend, they would not be able to deny you warranty for using another diesel fuel additive. The Federal Magnuson/Moss protects you there. Since a product, marketed as a diesel fuel additive is used, they cannot void warranty because you used another product properly for its intended purpose. However, the Magnuson/Moss act does not protect consumers who use a product for other than its marketed or intended purpose. The best example of this around these circles is guys who put 2 stroke gasoline mix oil in their diesel fuel. That product is intended for 2 stroke gasoline engines. It is not intended, by the manufacturer, for diesel use. Because of this "misuse" by definition, a manufacturer COULD rightly void warranty coverage for running 2 stroke oil, if they could determine that is what happened.
What's the point? Don't worry about using whatever diesel fuel additive works for you. I'm sure the stuff marketed by Fleetguard is fine. But I would caution guys who want to retain engine or fuel system warranty against using 2 stroke oil or some other product other than those intended and marketed as diesel fuel additives.