Some people are so desperate for attention they will come up with anything to interject themselves. You should have told the guy to take his kids to a park and pointed at the street and said, "This isnt a friend---ing jungle-gym sir, its asphalt designed for vehicles"
He probably went home and got drunk(er) and forgot all about why he had a picture of a UPS truck in his phone. But worst case, if he had the capacity to place a complaint that actually went through, the center would pull the telematics and check and see that you werent speeding. Id honestly bet that UPS gets this complaint all the time because we DONT drive at 8mph and it scares the hell out of old people seeing a huge truck doing the speedlimit or even a little under. I had a 400lb guy yell at me to slow down and I was legit doing 18mph on an unfinished, bumpy, new construction road. I just looked at him like he was an idiot and drove off. Didnt even bother to debate with him. To be fair though, he was so fat that 18mph must have seemed like light-speed travel to him