Correct. And in my limited understanding the 'blue' slider in the B&W conversion tool, for instance, affects a specific blue or range(?) of blue. Changing the saturation and hue changes how the 'blue' B&W slider affects the blues in the image. But yes, the luminance slider has the greatest impact.
For folks who are really deep into B&W tone control I recommend trying this approach. I have found it particularly useful in achieving a red filter effect to darken blue skies. Just applying the red filter in Silver Efex or the blue slider in LR will often result in banding and noise before the desired effect is reached. Using the HSL sliders gives a slight, but still significant level of control in darkening that blue without adding artifacts.