I haven't read the paper but it seems like it's saying rude prompts are better, so isn't it reasonable to assume that's what they meant? If we want to talk about directness, that's kind of a tangent right? I see directness as an entirely different dimension, you can be very direct and polite, you can be very rude and indirect (e.g. passive aggressive). Maybe they should do a follow-up study on how well AI responds based on level of directness.