I don't know if this was an attempt at condescension, but whatever. No kidding going over the speed limit will reduce reaction time. But if I literally step off the sidewalk into your car, your reaction time doesn't much matter now does it? Obviously, in that kind of case, you'd have the length of the skid marks analyzed and all sorts of things. But at the end of the day, it would be the idiot who walked into traffic without looking who CAUSED the accident...not the 5 mph over. Both are in the wrong, but only one is the cause of the accident. That was the point...and the comparison.
No, I wasn't being condescending. I was explaining it from the standpoint of the law and comparative fault (or comparative negligence). The trial for a situation like this would depend on exactly what you mentioned evidence of the skid marks, reaction times, etc. From the standpoint of the law both people are at fault: but for the pedestrian not paying attention the accident wouldn't have happened AND but for the driver hitting him with his car he wouldn't have been injured.
The law doesn't recognize who is "wrong" in a situation, only the causes. You may very will be right in your hypothetical that the pedestrian was the only cause of the incident. In fact, there's been cases that have gone both ways on this type of situation. Sometimes the pedestrian is at fault regardless of whether the driver was over the limit. Sometimes the driver is still apportioned a percentage of the fault, sometimes none. All those questions are things to be answered by the fact-finder in a case, either the judge or jury.