No idea, but is it possible that they were filming in a real, actual diner and asked for releases from people who were there at the time? Gave them screen credit when the scene really did end up in the film and people were visible?
Or possibly they put out an open call for extras to be in a scene and people just showed up if they wanted to do it? I've done that before, a loooooong time ago! Doesn't mean he's an actor on the side, just a lark probably.