747,
The statement that winglets produce thrust is not the same as saying that they produce or release their own energy. Winglets, like the sails of a boat extract energy from the airstream and use it to provide thrust. In the case of winglets this energy is of course initially released by the burning of fuel in the engines, but would otherwise be lost in the wingtip vortices. But the action of winglets is not simply a matter of reducing drag.
In an earlier post JT provided another example of this process in supersonic air intakes. It is often said that the convergent-divergent intakes of Concorde produced more than half of the thrust in supersoinc cruise. The engines alone could not produce this much thrust without using a far greater fuel flow.
But if we shut down the engines all of the thrust is lost. The engines need the intakes and the intakes need the engines. Neither alone can produce the extra thrust. But can we really say that the intakes simply reduce drag? No we cannot.
The intakes extract kinetic energy from the airflow and convert it into a form (static pressure) that increases the total thrust produced. But the energy was originally provided by the burning of fuel in the engines.
If winglets simply reduced drag then we could do this in a much more simple manner. Fitting external wing tip fuel tanks for example would restrict wingtip vortices and provide the additional benefit of reduced wing bending moments. Or we could fit simple end plates which would be structurally much more simple and less liable to fall off in flight.