That's not really proof of this. Randomly friending someone of the opposite gender is usually interpreted as a creepy attempt to pursue someone. If they're in a relationship, that motive has to be incorrect. Maybe it's someone you know from somewhere, or someone who likes your work. In that mindset, accepting the request is far more likely.
The way I understood this article is that women are more interested in guys who are not single whether they know him or not. This experiment will prove just that.
The experiment doesn't measure interest; it measures the likelihood of accepting a friend request. Accepting a friend request is not strong evidence of romantic interest due to the interactions I described.
