This is true, but what with sunlight being by far the largest factor in energy being radiated back from the surface of the earth you can safely conclude that in practice the greenhouse effect is to a large extent resulting in solar incident radiation being trapped. In other words: without sunlight nobody would be worrying about the greenhouse effect. In fact, without sunshine we'd love the greenhouse effect, reducing the albedo of the planet as a whole would be the only way to survive as a species (likely we'd fail).