It was serious dick move. Nothing to do with being nerd a lot to do with being jerk.
Seriously, I've been on tons of trade shows, both in the booth and as a visitor, I'd have definitely struck up a conversation with the guy to see where and how he learned so much about the device. That also would have all but guaranteed getting my device back in pristine working order and a pointer to some information about it that I apparently had missed.
So no, this was not about showing device capabilities. Nor about striking conversation. This was about feeling good and superior for making someone frustrated.
(And no, which is not even same as not caring about other peoples feelings, this is about being happy about their feelings being negative.)
Could the prankster have done better? Sure, no doubt, but it's not as bad as people make it out to be. Could the salesperson have done better? Yes, also no doubt, having been there this was simply a missed opportunity.
Perhaps I'm too young, but I've literally never know any such a salesperson. They're there to be charismatic and friendly, and show the features for which they've been handed a script.
Their job is to drive interest, and address very, VERY high level concerns. They're not experts, else they'd (by and large) be doing something other than sales, yes?
(To be fair, it's possible that we've just eliminated reasonable expectations of salespeople, but that's not clear to me yet)
But plenty of companies sent minders and order takers, technically incompetent, they might be able to do a scripted demo but likely would not know the first thing about the actual uses of such a device.
This was about 30-40 years ago, so it's far more in the time frame where you'd have technically competent sales folk, as you'd be pitching this gear to other engineers at this trade show.
When you're at a trade show for the very field that equipment is designed for and someone is trying to pitch you a piece of equipment that's nearly double the annual salary of an entry level engineer, I think you'd be far more likely to see a salesperson knowledgeable about their gear.
Let's have a bit of perspective here.