To my knowledge: for a knock on, the ball must move in the direction of the opposition's goalline. That doesn't seem to be the case here so I am okay with that try being awarded. I am not okay with the petulant behaviour shown by Biggar though. »
Basicly I agree with you. But just maybe Nigel Owens knows how Parisse must feel at this moment in the match and in his career and decides to take this into account and therefor lets this one (walking away) slip. If so, it makes him an even better, at least more human, referee in my opinion.