I think they specifically mentioned it does not affect it, someone asked that question in a webinar. If it would affect it, than it could be abused (eg. just send 10k bots that spend 1 hour on the site), plus a longer session doesn't necessarily mean a better site or better user experience. Google itself is a good example, where the point of Google is to spend as little time as possible on their page, as it's just a step between the user and his destination.