Someone watching a youtube video does not equate to a lost "sale" from a streaming service. The youtube viewer would have never paid to listen to your song.
What they are saying is that if the video with that copyrighted music was properly registered as that copyrighted work, Youtube would be paying a royalty for it or monetizing it for the record company/artist. Since it was uploaded by a user and not registered as the copyrighted work, Youtube does not pay for it until after it is discovered. It is not saying these were lost sales.
What they did not seem to mention was the knock-on effect of purchases made by people that heard the music in those unregistered videos that would likely bring down that $1 billion amount quite significantly.