Can't speak for the other commenter, but I'm "available" when I sit at my computer playing Civilization for 6 hours, despite the fact that I won't touch my IM application that entire time. If you're trying to set "Away" based on how often I interact with the application, you're getting it wrong for 5 hours and 40 minutes of that period.
"It’s not just about your usage of Chrome or a particular website: If you’ve stepped away from your computer and aren’t using any applications, Chrome can tell the website you’re not actively using your computer."
That suggests to me that this feature is more than just "did you touch that particular tab", and I'm not really sure how you're reading it differently?
