I just read the article. It nicely details the difference between physical lightness and perceptual brightness. For a demonstration of this, try opening a photo

I just read the article. It nicely details the difference between physical lightness and perceptual brightness. For a demonstration of this, try opening a photo of colorful flowers in Photoshop. Compare a de-saturation in RBG with one in Lab (Adjustments/Hu Saturation). The result from the Lab desaturation will respect the perceptual differences of the hue of the flowers flowers.