That's an interesting quotation, but it doesn't sound like the problem with Curio's was too much nuance. If anything, very plain (albeit clear) speech has a lack of nuance.
I don't think I've seen an author's point demonstrated so neatly in the comments ever before.
I think GP is trying to make the opposite point, that Calvus (not Curio) introduced too much nuance in his speech, making himself a less effective orator.
That whole paragraph is almost exclusively about Calvus; Curio is only mentioned once, in passing.