They were unremarkable in terms of the elbows Elon Musk rubs, so perhaps it was simply a first-clicked, first served affair. It wasn't even apparent if they drove home that day or simply took a ceremonial key and symbolic car.
Apparently only four customers were there (aside from 1000 onlooking customers, I didn't see any special situation for Sig buyers, so I guess that's an entirely separate dog and pony show.)
The only new X owner I recognized on stage last night was Steve Jurvetson.
I did hear Elon say, to a woman who went up on stage by herself to receive a key fob from him and who was then driven off the stage in a X, something to the effect that she was "a stand in for Sergey." My interpretation is that she took delivery of the car for Sergey Brin, one of the founders of Google.