Author wants someone who COMPLIMENTS them and can fit with their style to fill in description gaps, add more color to worlds, fill out characters better, and push up sex scenes.
This is not co-authoring. This is not a re-writing or editing, or changing of plot but adding. Like frosting to cake, not changing the cake.
Test is one un-edited chapter, per accepted word count pricing can be discussed if style meshes. Yes, it will be accepted words. Anyone can write a slew of adjectives, only usable additions with thought put into them will be paid. Good writers have nothing to worry on that.