The CDL transfert function in darktable is the standard one.
What you have seen, the luma correction, applies on the user input (interface) since we have HSL and RGBL interfaces, you want your HS/RGB parameters to affect only the color since you control the luminance separately. That would be super weird to control the module from the HSL interface and see the changes on HS affect the luminance.
The ASC CDL normalizes the transfert function, it doesn’t say anything on the UI implementation.
As for the saturation, darktable has other modules to control the saturation after the color balance. The interest of the saturation before the CDL is it allows split-tonings effects, by desaturating first, then changing the colors selectively.