It's an optical illusion. The planes aren't really that close together. The person who shot the video is using a telephoto lens and is zoomed way in. This compresses the space and flattens it out so it's hard to judge distance. Also the plane in front is smaller than the one in the back which heightens the illusion. It's a really cool shot!
Definitely the windows, but recent planes have gone with huge turbines so it's not the most reliable tell. I don't know Airbus well but the turbines on a 737-800 or Mac are pretty big compared to a - 300 or A320
It’s not as rare as you may think. I used to work at a weather service office located right near the end of one of the runways at IAD and it would happen a few times a day if the airport was busy and the winds were such that they were coming in from our side.