Thorium is a true mordant in the sense that the Azo-Sulfonate dyes precipitate as a Thorium salt. Today, Guanidinium salts and other positively charged quaternary ammonium salts are used as mordants.
As I understand it, the Thorium bound so tightly to the dyes that it does enhance sharpness relative to other mordants simply by tying the dye up strongly and quickly, preventing any bleed. Thorium also altered the hue of the cyan dye a tad. Jim probably has more on that.