I thought the question here was whether the acquantaince was rude. But I must admit I am stunned at the number of people who are supporting violations of the law.
Most of us aren't so perfect that we see driving a few miles over the speed limit as a particularly egregious sign of moral turpitude.
Yep. If that's the case, then toss me in jail and throw away the key. 
People are not simply saying "hey, not perfect, I admit I speed from time to time". They are basically maintaining that that makes them good drivers because they are keeping up with traffic or that they have superior judgement to the people who set the speed limit in the first place and thus are justified in disregarding it. I think it is that response that Kitty Hawk is pointing out. I have to say that when I do catch myself going over the speed limit I do not find myself saying "My, I am driving particularly safely today". If you have to justify yourself by saying you are less dangerous than a distracted driver that is not the best argument that you are actually safe, just that you are lower down on the "dangerous continuum".
I have a perfect driving record so far and I live in NJ. You've heard of a "NY minute"? They're as fast in Jersey as they are in NY. I'm being passed left and right, I don't tailgate or drive aggressively, and am a very safe driver. Being on the NJ Turnpike or the Garden State Parkway isn't the same as being on a quiet residential street (where I tend to go
below the speed limit due to kids/pets).My insurance rates are what they are because I am a safe driver.
Honestly, all of us quibbling over 5 mph seems a bit much. It's other things that make driver more dangerous such as tailgating, lane-jumping, using their cell phone or other distractions, and going real fast or too slowly.
My take: the guy in the OP overstepped.