Sorry, there's not enough information in the story to determine whether the door was transparent or not.
Good point. My front door, for example, has a 4-inch by 5-foot glass slot running vertically in the middle, so I can immediately see who's out there. I don't think it is that unusual.
Other doors have chains which allow them to be opened a few inches. It seems from the story that the homeowner identified the door banger as a threat by shouting through the door. I would like to know how THAT conversation went.