ryansutter.net

ABC AU didn't report the full story. He was inside the ISS when he threw the paper boomerang, so there was still air resistance.

http://entertainment.howstuffworks.com/boomerang2.htm

If he'd thrown it inside a vacuum, it wouldn't have worked. (AFAIK--and I am no expert--space isn't a "true" vacuum, but it's probably close enough the experiment wouldn't have worked if performed outside.)