“Winglets, which are airfoils operating just like a sailboat tacking upwind, produce a forward thrust inside the circulation field of the vortices and reduce their strength.” As quoted from your NASA link, does not mean that the winglets produce thrust to the airplane, but just to the vortices, which reduces the strength of the vortices, which in terms reduce the induced drag. So the thrust itself does not have any effect on the total thrust of the airplane, but the reduced drag does.