It seems that HSL is applied after rendering, while Channel Mixer before. That’s why HSL does not work with B&W conversions (it does not touch greys). You can also use FP Filters with B&W – they are applied before rendering.
It would be nice if HSL worked with B&W, because there is no Orange channel in the Mixer, which is important for skin tones. See Nik Collection or Filmpack - #18 by noir.fonce .