From the accounts here, it sounds like it's pretty easy to activate Summon when you just want to park your car, by accidentally pressing "park" twice instead of once. This is compounded by the fact that there is no "off" for Teslas, you're expected to park it and get out. Sure, there might be dings and dashboard lights, but I can easily see how someone might not see them, or might not know what they signify is about to happen. Finally, as far as I'm aware, there is no external sound or indication that the car is in Summon mode, it just starts to move, so I can easily see how, once you exit the vehicle, you'd be unaware of it happening behind you.
Activating Summon by double-clicking the Park button is an optional feature that's off by default. In addition, Summon itself is an optional feature that's off by default. Both settings come with warnings about appropriate use. So while the feature is fairly easy to activate once you have the settings enabled, the easy answer to it is to leave it disabled.
Is it possible that those safeguards were deactivated by the Tesla delivery specialist, or by the customer at the suggestion of the delivery specialist, in order to demonstrate how the feature works? I genuinely don't know if that's something the delivery specialists do, but I think it's plausible. Tesla demonstrates Autopilot on test drives as a matter of course, and I could easily see them wanting to demonstrate Summon to new customers.
If so, I think it's possible the customer was not made aware of the safety implications and limitations of the system.
It's possible, in that it's not against the laws of physics or anything. I've never heard of anyone having such things enabled by anyone from Tesla, though, and I suspect that if they did it for this person, he'd be telling everyone who will listen.
