Rawpedia says:
The Lab Chromaticity slider increases or decreases the chromaticity of the image, by applying a contrast curve to the a- and b-channels of Lab space. Setting this slider to -100 removes all color, making the image black and white. The best way to convert an image to black-and-white is by using the dedicated and powerful Black-and-White tool in the Color tab.
http://rawpedia.rawtherapee.com/Lab_Adjustments#Chromaticity
This sounds like you have to go to the ‘tone curve’ module, set it to ‘LAB independent channel’ and add contrast to A and B channels.
(note that I am not a Rawtherapee user and may be wrong here)