It found my unique Imgur username. So I guess that is where he gets the username from.

If you're curious on how he "finds out", check out his other video (Quickjack - Hacking Facebook likes with Clickjacking): https://www.youtube.com/watch?v=bCkSVGhIEb4#t=217

Very much so. Seeing my user name populated in the bottom task bar was a little unnerving.

Being marked 'you have nothing to worry about' was somehow equally as unnerving.

I love that 'desktop as website' theme, gonna steal it.

Oh, I do know pwnat. It could be quite useful.

edit: It seems like this guy really likes to show how useless NAT is. His "NAT pinning" demonstration is quite scary.