HSLuv starts with a cylindrical representation of CIELUV which also doesn't "fill the cylinder" in the chroma axis (this might be what the article briefly refers to as "HCL or LCh(uv)"), and then stretches its chroma so that you always get a 0-100% chroma range regardless of hue and lightness. This might be convenient because you can't accidentally represent impossible colors, but you obviously lose perceptual chroma uniformity.