Yes, something like that. I think it’d make sense the make the ‘saturation’ arrow curved (it is my understanding that distance from the black point is constant). I also liked the wording ‘The lines of equal saturation are the oblique ones, so the saturation setting opens or closes their angle like a flower, while purity [brilliance] moves along those oblique lines.’ I just like the mental image it conjures up. 
The specific post was this: Editing moments with darktable - #207 by aurelienpierre