I hate how every discussion of bits ends up in the incredibly-not-useful statement of "Even a snaffle in the wrong hands can be harsh" :eyeroll:
It sounds like the person telling you this was basically saying that the metal noseband pictured is SO harsh that the horse won't pull against it AT ALL. A rope is going to have more friction, so probably is more likely to rub the horse raw, but I don't think there's any question that that metal noseband is highly uncomfortable. You'd have to define what type of rope noseband you're comparing against (thickness, material, etc.) to say which is truly harsher.